Remove an explanation about example as it is self-explanatory

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

This paves the way for installing other roles into `roles/galaxy` using `ansible-galaxy`,
similar to how it's done in:

- https://github.com/spantaleev/gitea-docker-ansible-deploy
- https://github.com/spantaleev/nextcloud-docker-ansible-deploy

In the near future, we'll be removing a lot of the shared role code from here
and using upstream roles for it. Some of the core `matrix-*` roles have
already been extracted out into other reusable roles:

- https://github.com/devture/com.devture.ansible.role.postgres
- https://github.com/devture/com.devture.ansible.role.systemd_docker_base
- https://github.com/devture/com.devture.ansible.role.timesync
- https://github.com/devture/com.devture.ansible.role.vars_preserver
- https://github.com/devture/com.devture.ansible.role.playbook_runtime_messages
- https://github.com/devture/com.devture.ansible.role.playbook_help

We just need to migrate to those.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2076

Doing this further simplifies examples/vars.yml.

We're trying to move away from implementation-specific variables,
hoping for a clean (implementation-neutral) examples/vars.yml file.

Doing this seems more reasonable and simpler.

In short, this makes Synapse a 2nd class citizen,
preparing for a future where it's just one-of-many homeserver software
options.

We also no longer have a default Postgres superuser password,
which improves security.

The changelog explains more as to why this was done
and how to proceed from here.

Prompted by this: https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/779#issuecomment-764807507

With this change, the following roles are now only dependent
on the minimal `matrix-base` role:
- `matrix-corporal`
- `matrix-coturn`
- `matrix-mailer`
- `matrix-mxisd`
- `matrix-postgres`
- `matrix-riot-web`
- `matrix-synapse`

The `matrix-nginx-proxy` role still does too much and remains
dependent on the others.

Wiring up the various (now-independent) roles happens
via a glue variables file (`group_vars/matrix-servers`).
It's triggered for all hosts in the `matrix-servers` group.

According to Ansible's rules of priority, we have the following
chain of inclusion/overriding now:
- role defaults (mostly empty or good for independent usage)
- playbook glue variables (`group_vars/matrix-servers`)
- inventory host variables (`inventory/host_vars/matrix.<your-domain>`)

All roles default to enabling their main component
(e.g. `matrix_mxisd_enabled: true`, `matrix_riot_web_enabled: true`).
Reasoning: if a role is included in a playbook (especially separately,
in another playbook), it should "work" by default.

Our playbook disables some of those if they are not generally useful
(e.g. `matrix_corporal_enabled: false`).

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.

Fix typo
Move mautrix variable in `defaults/main.yml` exclusively

Until now, we were starting from a fresh configuration, as generated
by Synapse and manipulating it with regex and line replacements,
until we made it work.

This is more fragile and less predictable, so we're moving to a static
configuration file generated from a Jinja template.

The upside is that configuration will be stable and predictable.

The downside of this new approach is that any manual configuration changes
after the playbook is done, will be thrown away on future playbook
invocations.

There are 2 ways to work around the need for manual configuration
changes though:
- making them part of this playbook and its default template
configuration files (which benefits everyone)
- going your own way for a given host and overriding the template files
that gets used (that is, the
`matrix_synapse_template_synapse_homeserver` or
`matrix_synapse_template_synapse_log` variables)

Switching from from avhost/docker-matrix (silviof/docker-matrix)
to matrixdotorg/synapse.

The avhost/docker-matrix (silviof/docker-matrix) image used to bundle
in the coturn STUN/TURN server, so as part of the move,
we're separating this to a separately-ran service
(matrix-coturn.service, powered by instrumentisto/coturn-docker-image)

Moving keeps everything in the /matrix directory, so that we
wouldn't contaminate anything else on the system or risk
clashing with something else.

Also retrieving certificates separately for the Riot and Matrix domains,
which should help in multiple ways:

- allows them to be very different (completely separate base domain..)

- allows for Riot to be disabled for the playbook some time later
  and still have the code not break