This also adds labels for Synapse. Support for other homeservers and
components will be added later.

This also updates validation tasks and documentation, pointing to
variables in the matrix-synapse role which don't currently exist yet
(e.g. `matrix_synapse_container_labels_client_synapse_admin_api_enabled`).

These variables will be added soon, as Traefik labels are added to the
`matrix-synapse` role. At that point, the `matrix-synapse-reverse-proxy-companion` role
will be updated to also use them.

Rate limit docs had a issue due to the wrong request type by accident. This was not noticed due to both bots can work with rate limits. Synapse Admin links where also bugged but now fixed.