Docs: include syn2mas docker option

*This issue was originally created by [**@joepitt91**](https://github.com/joepitt91) at .*

After a bit of trail and error I've successfully migrated my synapse server that was using `oidc_providers:` to use `experimental_features: msc3861:`.

The only real feedback I've got on the process is that it would be good if the docs pointed to the docker container for `syn2mas` and how to use it as I didn't find this bit clear, especially how to map old OIDC realm to new MAS realm.

Suggested commands for docs:

```sh
# Alias syn2mas for readability and ease of typing
alias syn2mas='docker run --rm -v /srv/docker/matrix/mas-config.yaml:/mas.yml -v /srv/docker/matrix/server/homeserver.yaml:/homeserver.yaml ghcr.io/matrix-org/matrix-authentication-service/syn2mas'

# Run the advisor
syn2mas --command advisor --synapseConfigFile /homeserver.yaml

# Do a dry run and check output
syn2mas --command migrate --synapseConfigFile /homeserver.yaml --masConfigFile /mas.yaml --upstreamProviderMapping oidc-keycloak:00000000000000000000000000 --dryRun

# Stop the server
docker compose down

# Do the migration
syn2mas --command migrate --synapseConfigFile /homeserver.yaml --masConfigFile /mas.yaml --upstreamProviderMapping oidc-keycloak:00000000000000000000000000 --dryRun false

# Remove the alias as it is no longer needed
unalias syn2mas

# Bring the server back up
docker compose up -d
```

In case it is useful for the docs too, this is my current `docker-compose.yml` which seems to work well - upstream HTTPS reverse proxy then proxies to 10.0.0.1:80 and 10.0.0.1:8080 for matrix.domain.tld and mas.domain.tld respectively.

```yaml
---
services:
client:
container_name: matrix-element
depends_on:
server:
condition: service_healthy
healthcheck:
test: wget --no-verbose --tries=1 --spider http://127.0.0.1/#/welcome || exit 1
image: vectorim/element-web:latest
restart: unless-stopped
volumes:
- ./client.json:/app/config.json
proxy:
container_name: matrix-reverse-proxy
depends_on:
client:
condition: service_healthy
mas:
condition: service_healthy
server:
condition: service_healthy
sliding-sync:
condition: service_healthy
healthcheck:
test: curl --fail http://127.0.0.1/_matrix/client/versions || exit 1
image: nginx:1.25
ports:
- 10.0.0.1:80:80/tcp
restart: unless-stopped
volumes:
- ./proxy.conf:/etc/nginx/conf.d/default.conf:ro
server:
container_name: matrix-server
environment:
- SYNAPSE_CONFIG_PATH=/data/homeserver.yaml
image: ghcr.io/element-hq/synapse:latest
restart: unless-stopped
volumes:
- ./server:/data
sliding-sync:
container_name: matrix-sync-server
depends_on:
server:
condition: service_healthy
healthcheck:
# best option found so far
test: if [ "$(curl --write-out '%{http_code}' --silent --output /dev/null http://127.0.0.1:8008/_matrix/client/unstable/org.matrix.msc3575/sync)" = "405" ]; then exit 0; else exit 1; fi
environment:
- SYNCV3_SERVER=http://server:8008
- SYNCV3_SECRET=[redcated]
- SYNCV3_BINDADDR=:8008
- SYNCV3_DB=user=synapse dbname=slidingsync sslmode=require host=[redacted] password='[redacted]'
image: ghcr.io/matrix-org/sliding-sync:latest
restart: unless-stopped
mas:
container_name: matrix_mas
healthcheck:
test: ["CMD", "mas-cli", "config", "check"]
image: ghcr.io/matrix-org/matrix-authentication-service:main
ports:
- 10.0.0.1:8080:8080
restart: unless-stopped
volumes:
- ./mas-config.yaml:/config.yaml
```

Hopefully this is useful feedback 🤞