Move roles/matrix* to roles/custom/matrix*

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.

docs/configuring-playbook-ntfy.md

@@ -23,7 +23,7 @@ matrix_ntfy_configuration_extension_yaml: |
   log_level: DEBUG
 ```
 
-For a more complete list of variables that you could override, see `roles/matrix-ntfy/defaults/main.yml`.
+For a more complete list of variables that you could override, see `roles/custom/matrix-ntfy/defaults/main.yml`.
 
 For a complete list of ntfy config options that you could put in `matrix_ntfy_configuration_extension_yaml`, see the [ntfy config documentation](https://ntfy.sh/docs/config/#config-options).