Auto-enabling the OIDC APIs is convenient for people
using the new `matrix_synapse_oidc_*` variables.

This change adds a section that lets the reader know that it is possible to enable synapse metrics and references the relevant documentation.

It aims to be short and concise, leaving the specifics to the referenced documentation, while still giving the reader a good idea of what it is and what it is for.

Similar to:
362954aeab

This warning was added because of:
https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2090

The problem has since been fixed by:
e9e84341a9

This current patch was provoked by:
https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2352

I'm not sure, but this should be changed to devture_postgres_... !?
https://github.com/spantaleev/matrix-docker-ansible-deploy/blob/master/CHANGELOG.md#2022-11-28

```yaml
matrix_postgres_process_extra_arguments: [
  "-c 'max_connections=200'"
]
```

Use up-to-date example from synapse docs

Add link to keycloak website

Add link to synapse docs on OIDC

This paves the way for installing other roles into `roles/galaxy` using `ansible-galaxy`,
similar to how it's done in:

- https://github.com/spantaleev/gitea-docker-ansible-deploy
- https://github.com/spantaleev/nextcloud-docker-ansible-deploy

In the near future, we'll be removing a lot of the shared role code from here
and using upstream roles for it. Some of the core `matrix-*` roles have
already been extracted out into other reusable roles:

- https://github.com/devture/com.devture.ansible.role.postgres
- https://github.com/devture/com.devture.ansible.role.systemd_docker_base
- https://github.com/devture/com.devture.ansible.role.timesync
- https://github.com/devture/com.devture.ansible.role.vars_preserver
- https://github.com/devture/com.devture.ansible.role.playbook_runtime_messages
- https://github.com/devture/com.devture.ansible.role.playbook_help

We just need to migrate to those.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2090

Added an example for configuring the variables for the OpenID, that might fails because the variables are not defined for the playbook parsing, but Synapse's own config file parsing.

Adding more presets in the future would be nice.

This broke in 63a49bb2dc.

Proxying the OpenID Connect endpoints is now possible,
but needs to be enabled explicitly now.

Supersedes #702 (Github Pull Request).

This patch builds up on the idea from that Pull Request,
but does things in a cleaner way.

Fixes #562 (Github Issue)