Improvement of documentation

Co-authored-by: Jan <31133207+Jaffex@users.noreply.github.com>

Document how to enable relay functionality in a room

Co-authored-by: Jan <31133207+Jaffex@users.noreply.github.com>

The default of using the `matrix` database is better anyway.

The attached code for the "Serving the Federation API with your certificates and matrix-nginx-proxy" section suggests using the matrix.<your-domain> certificate for the federation API as opposed to the necessary <your-domain> certificate for the federation to work. This can cause some confusion to readers.

Recently, documentation on Synapse has been changed from .rst to .md. Therefore, the current links for the purge history API were resulting in a 404 error.

These previously denoted optional DNS records but now optional records are a separate table so they are unnecessary.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1131

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.

This commit introduces a new role that downloads and installs the
prometheus community postgres exporter  https://github.com/prometheus-community/postgres_exporter.
A new credential is added to matrix_postgres_additional_databases that
allows the exporter access to the database to gather statistics.
A new dashboard was added to the grafana role, with some refactoring
to enable the dashboard only if the new role is enabled.
I've included some basic instructions for how to enable the role in
the Docs section.

In terms of testing, I've tested enabling the role, and disabling
it to make sure it cleans up the container and systemd role.

Annotate server migration with a note in regard to migration between different architectures and the need to dump/restore the database.

Added a mid-sized VPS configuration with configuration changes to the PostgreSQL database config.

Deleted single quotes in one of the examples to unify the examples

# chain.pem file is required for 'ssl_trusted_certificate`

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1055

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1055

Fix https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1073

Related to https://github.com/Awesome-Technologies/synapse-admin/issues/132