This adopts the structure of docs/configuring-playbook-matrix-authentication-service.md which was recently created.

- …
- Adjusting the playbook configuration
  - …
  - Adjusting the (service name here) URL
  - …
- Adjusting DNS records
- Installing
- …

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

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/683