Add a role to install 'ntfy' push-notification server.

This commit adds a 'matrix-ntfy' role that runs Ntfy server in Docker with
simple configuration, and plumbing to add the role to the playbook.

TODO: documentation, self-check, database persistence.

roles/matrix-ntfy/README.md

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+# A role to install the [ntfy](https://ntfy.sh) push-notification server.
+
+The ntfy server and clients implement self-hosted support push notifications
+from Matrix (and other) servers to Android (and other) clients, using the
+[UnifiedPush](https://unifiedpush.org) standard.
+
+This role installs ntfy server in Docker.  It is intended to support push
+notifications, via UnifiedPush, from the Matrix and Matrix-related services
+that are installed alongside it to any clients that support UnifiedPush.
+
+This role is not intended to support other features of the ntfy server and
+clients.
+
+
+# Using the ntfy role
+
+Configure the role by adding settings in your Ansible inventory.
+
+The only required setting is to enable ntfy:
+
+    matrix_ntfy_enabled: true
+
+The default domain for ntfy is `ntfy.<matrix_domain>`.  This can be changed
+with the `matrix_server_fqn_ntfy` variable:
+
+    matrix_server_fqn_ntfy: "my-ntfy.{{ matrix_domain }}"
+
+Other ntfy settings can be configured by adding extra arguments to the
+docker run command, e.g.:
+
+    matrix_ntfy_container_extra_arguments:
+      - '--env=NTFY_LOG_LEVEL=DEBUG'
+
+
+# TODO
+
+- Documentation.
+- Self-check.
+- Mount the ntfy database to disk so subscriptions persist across restarts.
+- Authentication?