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-nginx.md

@@ -42,7 +42,7 @@ Besides changing the preset (`matrix_nginx_proxy_ssl_preset`), you can also dire
 - `matrix_nginx_proxy_ssl_prefer_server_ciphers`: for specifying if the server or the client choice when negotiating the cipher. It can set to `on` or `off`.
 - `matrix_nginx_proxy_ssl_ciphers`: for specifying the SSL Cipher suites used by nginx.
 
-For more information about these variables, check the `roles/matrix-nginx-proxy/defaults/main.yml` file.
+For more information about these variables, check the `roles/custom/matrix-nginx-proxy/defaults/main.yml` file.
 
 ## Synapse + OpenID Connect for Single-Sign-On
 
@@ -80,5 +80,5 @@ matrix_nginx_proxy_proxy_http_additional_server_configuration_blocks:
     # These lines will be included in the nginx configuration.
     # This is at the top level of the file, so you will need to define all of the `server { ... }` blocks.
   - |
-    # For advanced use, have a look at the template files in `roles/matrix-nginx-proxy/templates/nginx/conf.d`
+    # For advanced use, have a look at the template files in `roles/custom/matrix-nginx-proxy/templates/nginx/conf.d`
 ```