This change should make it more obvious which order the steps must be done

Also indicates that the Mautrix googlechat bridge is a replacement for hangouts.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1322

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1282

It's mostly due to Docker CE dropping its repositories (and support) for
Debian 9.
If one installs Docker manually (likely a package named `docker.io`), it
will likely still work.
In any case, Debian 9 is old and end-of-life now, so advertising support
for it is not productive.

Ubuntu ended support for 16.04 in April

This avoids having to create a new room and to click the "Add widgets, bridges & bots" link (Formerly the four-squares-icon)

matrix_jitsi_jicofo_component_secret var removed #1139

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1242

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.