Update OIDC Authentik documentation to mention required `jwt_config` Synapse configuration

### Description

When using the official guides to connect Synapse to authentik (https://element-hq.github.io/synapse/latest/openid.html#authentik and https://docs.goauthentik.io/integrations/services/matrix-synapse/) Synapse raises a `JSONDecodeError` when trying to login.

Steps to reproduce:
- Setup Synapse and authentik as described in the links above.
- Navigate to Element-Web, click on "Login with authentik".
- Provide valid credentials. authentik now redirects to Synapse.
- Synapse shows an error within the web browser: "Internal Server Error".

```
[...]
synapse-1 | 2024-11-03T17:23:17.988935881Z 2024-11-03 17:23:17,988 - synapse.rest.client.login - 682 - INFO - GET-7 - Redirecting to https://auth.REDACTED.de/application/o/authorize/?response_type=code&client_id=WRKvT2yhGvUeSGf9AJshP3vjhgwR6tj2BtYbDVHF&redirect_uri=https%3A%2F%2Fmatrix.REDACTED.de%2F_synapse%2Fclient%2Foidc%2Fcallback&scope=openid+profile+email&state=uw0yDmOCuR1lW4dBL9BjLTBCZfSnr6&nonce=0nbfV9h07yVd9emtq54HVqrDfEs8Vy&code_challenge_method=S256&code_challenge=colWx9z0kYx1YX8ceBteeAiEVm7PAdZfXa-Ko_Iizy0
synapse-1 | 2024-11-03T17:23:17.989612259Z 2024-11-03 17:23:17,989 - synapse.access.http.8008 - 473 - INFO - GET-7 - 172.18.0.1 - 8008 - {None} Processed request: 0.004sec/-0.000sec (0.004sec, 0.000sec) (0.000sec/0.000sec/0) 0B 302 "GET /_matrix/client/v3/login/sso/redirect/oidc-authentik?redirectUrl=https%3A%2F%2Felement.REDACTED.de%2F&org.matrix.msc3824.action=login HTTP/1.1" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/18.1 Safari/605.1.15" [0 dbevts]
synapse-1 | 2024-11-03T17:23:18.659634171Z 2024-11-03 17:23:18,659 - synapse.handlers.oidc - 259 - INFO - GET-8 - Received OIDC callback for IdP oidc-authentik
synapse-1 | 2024-11-03T17:23:19.140305318Z 2024-11-03 17:23:19,137 - synapse.http.client - 428 - INFO - GET-8 - Received response to POST https://auth.REDACTED.de/application/o/token/: 405
synapse-1 | 2024-11-03T17:23:19.152399538Z 2024-11-03 17:23:19,138 - synapse.http.server - 221 - ERROR - GET-8 - Failed handle request
synapse-1 | 2024-11-03T17:23:19.152457450Z Traceback (most recent call last):
synapse-1 | 2024-11-03T17:23:19.152466090Z File "/usr/local/lib/python3.11/site-packages/synapse/http/server.py", line 332, in _async_render_wrapper
synapse-1 | 2024-11-03T17:23:19.152473300Z callback_return = await self._async_render(request)
synapse-1 | 2024-11-03T17:23:19.152480002Z ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
synapse-1 | 2024-11-03T17:23:19.152486398Z File "/usr/local/lib/python3.11/site-packages/synapse/http/server.py", line 364, in _async_render
synapse-1 | 2024-11-03T17:23:19.152493234Z callback_return = await raw_callback_return
synapse-1 | 2024-11-03T17:23:19.152499662Z ^^^^^^^^^^^^^^^^^^^^^^^^^
synapse-1 | 2024-11-03T17:23:19.152506064Z File "/usr/local/lib/python3.11/site-packages/synapse/rest/synapse/client/oidc/callback_resource.py", line 42, in _async_render_GET
synapse-1 | 2024-11-03T17:23:19.152512890Z await self._oidc_handler.handle_oidc_callback(request)
synapse-1 | 2024-11-03T17:23:19.152520274Z File "/usr/local/lib/python3.11/site-packages/synapse/handlers/oidc.py", line 276, in handle_oidc_callback
synapse-1 | 2024-11-03T17:23:19.152527014Z await oidc_provider.handle_oidc_callback(request, session_data, code)
synapse-1 | 2024-11-03T17:23:19.152533634Z File "/usr/local/lib/python3.11/site-packages/synapse/handlers/oidc.py", line 1094, in handle_oidc_callback
synapse-1 | 2024-11-03T17:23:19.152540384Z token = await self._exchange_code(
synapse-1 | 2024-11-03T17:23:19.152546618Z ^^^^^^^^^^^^^^^^^^^^^^^^^^
synapse-1 | 2024-11-03T17:23:19.152552859Z File "/usr/local/lib/python3.11/site-packages/synapse/handlers/oidc.py", line 787, in _exchange_code
synapse-1 | 2024-11-03T17:23:19.152559212Z resp = json_decoder.decode(resp_body.decode("utf-8"))
synapse-1 | 2024-11-03T17:23:19.152565486Z ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
synapse-1 | 2024-11-03T17:23:19.152571802Z File "/usr/local/lib/python3.11/json/decoder.py", line 337, in decode
synapse-1 | 2024-11-03T17:23:19.152578139Z obj, end = self.raw_decode(s, idx=_w(s, 0).end())
synapse-1 | 2024-11-03T17:23:19.152584650Z ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
synapse-1 | 2024-11-03T17:23:19.152590962Z File "/usr/local/lib/python3.11/json/decoder.py", line 355, in raw_decode
synapse-1 | 2024-11-03T17:23:19.152597458Z raise JSONDecodeError("Expecting value", s, err.value) from None
synapse-1 | 2024-11-03T17:23:19.152603926Z json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
synapse-1 | 2024-11-03T17:23:19.153510452Z 2024-11-03 17:23:19,153 - synapse.access.http.8008 - 473 - INFO - GET-8 - 172.18.0.1 - 8008 - {None} Processed request: 0.495sec/-0.000sec (0.011sec, 0.000sec) (0.000sec/0.000sec/0) 167B 500 "GET /_synapse/client/oidc/callback?code=570eb483fd9242f5b1be01f2ba70002c&state=uw0yDmOCuR1lW4dBL9BjLTBCZfSnr6 HTTP/1.1" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/18.1 Safari/605.1.15" [0 dbevts]
[...]
```

Relevant config snippet from `homeserver.yaml`:
```
[...]
oidc_providers:
- idp_id: authentik
idp_name: authentik
discover: true
issuer: "https://auth.REDACTED.de/application/o/synapse-slug/"
client_id: "REDACTED}"
client_secret: "REDACTED"
scopes:
- "openid"
- "profile"
- "email"
user_mapping_provider:
config:
localpart_template: "{{ user.preferred_username }}"
display_name_template: "{{ user.name|capitalize }}"
[...]
```

### Steps to reproduce

- Setup Synapse and authentik as described in the links above.
- Navigate to Element-Web, click on "Login with authentik".
- Provide valid credentials. authentik now redirects to Synapse.
- Synapse shows an error within the web browser: "Internal Server Error".

### Homeserver

Self-hosted homeserver

### Synapse Version

v1.118.0

### Installation Method

Docker (matrixdotorg/synapse)

### Database

PostgreSQL, single server, no ported data (fresh set up)

### Workers

Single process

### Platform

Debian 12 VM via Docker

### Configuration

_No response_

### Relevant log output

```shell
[...]
synapse-1 | 2024-11-03T17:23:17.988935881Z 2024-11-03 17:23:17,988 - synapse.rest.client.login - 682 - INFO - GET-7 - Redirecting to https://auth.REDACTED.de/application/o/authorize/?response_type=code&client_id=WRKvT2yhGvUeSGf9AJshP3vjhgwR6tj2BtYbDVHF&redirect_uri=https%3A%2F%2Fmatrix.REDACTED.de%2F_synapse%2Fclient%2Foidc%2Fcallback&scope=openid+profile+email&state=uw0yDmOCuR1lW4dBL9BjLTBCZfSnr6&nonce=0nbfV9h07yVd9emtq54HVqrDfEs8Vy&code_challenge_method=S256&code_challenge=colWx9z0kYx1YX8ceBteeAiEVm7PAdZfXa-Ko_Iizy0
synapse-1 | 2024-11-03T17:23:17.989612259Z 2024-11-03 17:23:17,989 - synapse.access.http.8008 - 473 - INFO - GET-7 - 172.18.0.1 - 8008 - {None} Processed request: 0.004sec/-0.000sec (0.004sec, 0.000sec) (0.000sec/0.000sec/0) 0B 302 "GET /_matrix/client/v3/login/sso/redirect/oidc-authentik?redirectUrl=https%3A%2F%2Felement.REDACTED.de%2F&org.matrix.msc3824.action=login HTTP/1.1" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/18.1 Safari/605.1.15" [0 dbevts]
synapse-1 | 2024-11-03T17:23:18.659634171Z 2024-11-03 17:23:18,659 - synapse.handlers.oidc - 259 - INFO - GET-8 - Received OIDC callback for IdP oidc-authentik
synapse-1 | 2024-11-03T17:23:19.140305318Z 2024-11-03 17:23:19,137 - synapse.http.client - 428 - INFO - GET-8 - Received response to POST https://auth.REDACTED.de/application/o/token/: 405
synapse-1 | 2024-11-03T17:23:19.152399538Z 2024-11-03 17:23:19,138 - synapse.http.server - 221 - ERROR - GET-8 - Failed handle request
synapse-1 | 2024-11-03T17:23:19.152457450Z Traceback (most recent call last):
synapse-1 | 2024-11-03T17:23:19.152466090Z File "/usr/local/lib/python3.11/site-packages/synapse/http/server.py", line 332, in _async_render_wrapper
synapse-1 | 2024-11-03T17:23:19.152473300Z callback_return = await self._async_render(request)
synapse-1 | 2024-11-03T17:23:19.152480002Z ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
synapse-1 | 2024-11-03T17:23:19.152486398Z File "/usr/local/lib/python3.11/site-packages/synapse/http/server.py", line 364, in _async_render
synapse-1 | 2024-11-03T17:23:19.152493234Z callback_return = await raw_callback_return
synapse-1 | 2024-11-03T17:23:19.152499662Z ^^^^^^^^^^^^^^^^^^^^^^^^^
synapse-1 | 2024-11-03T17:23:19.152506064Z File "/usr/local/lib/python3.11/site-packages/synapse/rest/synapse/client/oidc/callback_resource.py", line 42, in _async_render_GET
synapse-1 | 2024-11-03T17:23:19.152512890Z await self._oidc_handler.handle_oidc_callback(request)
synapse-1 | 2024-11-03T17:23:19.152520274Z File "/usr/local/lib/python3.11/site-packages/synapse/handlers/oidc.py", line 276, in handle_oidc_callback
synapse-1 | 2024-11-03T17:23:19.152527014Z await oidc_provider.handle_oidc_callback(request, session_data, code)
synapse-1 | 2024-11-03T17:23:19.152533634Z File "/usr/local/lib/python3.11/site-packages/synapse/handlers/oidc.py", line 1094, in handle_oidc_callback
synapse-1 | 2024-11-03T17:23:19.152540384Z token = await self._exchange_code(
synapse-1 | 2024-11-03T17:23:19.152546618Z ^^^^^^^^^^^^^^^^^^^^^^^^^^
synapse-1 | 2024-11-03T17:23:19.152552859Z File "/usr/local/lib/python3.11/site-packages/synapse/handlers/oidc.py", line 787, in _exchange_code
synapse-1 | 2024-11-03T17:23:19.152559212Z resp = json_decoder.decode(resp_body.decode("utf-8"))
synapse-1 | 2024-11-03T17:23:19.152565486Z ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
synapse-1 | 2024-11-03T17:23:19.152571802Z File "/usr/local/lib/python3.11/json/decoder.py", line 337, in decode
synapse-1 | 2024-11-03T17:23:19.152578139Z obj, end = self.raw_decode(s, idx=_w(s, 0).end())
synapse-1 | 2024-11-03T17:23:19.152584650Z ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
synapse-1 | 2024-11-03T17:23:19.152590962Z File "/usr/local/lib/python3.11/json/decoder.py", line 355, in raw_decode
synapse-1 | 2024-11-03T17:23:19.152597458Z raise JSONDecodeError("Expecting value", s, err.value) from None
synapse-1 | 2024-11-03T17:23:19.152603926Z json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
synapse-1 | 2024-11-03T17:23:19.153510452Z 2024-11-03 17:23:19,153 - synapse.access.http.8008 - 473 - INFO - GET-8 - 172.18.0.1 - 8008 - {None} Processed request: 0.495sec/-0.000sec (0.011sec, 0.000sec) (0.000sec/0.000sec/0) 167B 500 "GET /_synapse/client/oidc/callback?code=570eb483fd9242f5b1be01f2ba70002c&state=uw0yDmOCuR1lW4dBL9BjLTBCZfSnr6 HTTP/1.1" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/18.1 Safari/605.1.15" [0 dbevts]
[...]
```

### Anything else that would be useful to know?

_No response_

Start by comparing the Synapse Authentik guide and the authentik Matrix Synapse guide, then search the repository for the OIDC Authentik documentation and the homeserver.yaml example. Document the required jwt_config configuration so the described setup does not fail during the OIDC callback with JSONDecodeError; verify the resulting guidance against the listed reproduction steps.

Written by the indexing model from the issue text.