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.

the old one is deprecated (says the page at the link target), and refers to this new one.

* Add example configuration for Caddy v2

Add a basic example how to get synapse-admin running behind Caddy v2 proxy.

* Improve working, fix typos

* Fix typos

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

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

Regression since 63a49bb2dc.

Related to #685 (Github Issue).

We now automatically expose the APIs when Synapse Admin is enabled.

Fixes #562 (Github Issue)