The different configurations are now all lower case, for consistent
naming.

`matrix_nginx_proxy_ssl_config` is now called
`matrix_nginx_proxy_ssl_preset`. The different options for "modern",
"intermediate" and "old" are stored in the main.yml file, instead of
being hardcoded in the configuration files. This will improve the
maintainability of the code.

The "custom" preset was removed. Now if one of the variables is set, it
will use it instead of the preset. This will allow to mix and match more
easily, for example using all the intermediate options but only
supporting TLSv1.2. This will also provide better backward
compatibility.

The new variables created to the nginx reverse proxy are properly added
to the documentation.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/735

Fixes #586 (Github Issue)

Use playbook variables instead of textual place-holders for the domain names.

Discussed in #412 (Github Issue).

This supersedes #59 (Github Pull Request),
which was greatly beneficial in creating our sample Apache configuration.

This was mentioned in #27 (Github Pull Request),
but it's just now that the nginx Docker image actually supports
TLS v1.3 and we can enable it.

Adds support for managing certificates manually and for
having the playbook generate self-signed certificates for you.

With this, Let's Encrypt usage is no longer required.

Fixes Github issue #50.

We've had some people get confused into installing
Matrix Corporal and having pain with that.

With this documentation change, we try to make it clearer
that it's an advanced feature not to be touched unless
you know what you're doing.

On a similar note, we also make sure other things are properly
labeled as "(optional)" and/or "(advanced)".

It's been incorrect since 23e4a473