Missing OIDC client documentation on custom claims

# Problem

The documentation with examples on the website is excellent, yet I cannot find any details how to retrieve custom claims.
I think this may just have been forgotten? Or one is supposed to know intuitively?

I followed the example for the Flask OIDC client and can get all the standard claims.
```python
oauth.register(
"organization",
server_metadata_url="[...]",
client_kwargs={"scope": "openid email profile"},
)

@bp.route("/auth")
def auth():
token = oauth.organization.authorize_access_token()
session["user"] = token["userinfo"]
```

However there is no documentation for custom claims, as shown below.

# Proposed solution

Given a well known config with

```
"claims_supported": [
"address",
"birthdate",
"client_id",
"cn",
"company",
"department",
...
]
```

add example code to the documentation at on how to retrieve the custom claim for department for instance.

I have looked around the issues here, looked on Stack Overflow, nada.
I can make a contribution and extend the examples, when someone can help me with this.