flatten the "playbooks" directory for better structure

Because of how Ansible local relative search paths work, the global
"files" and "templates" directories need to be next to the playbooks.
However its not intuitive to look into the playbooks directory to find
the files and templates for a host.
Therefore flatten the playbooks directory to get rid of this confusing
structure.

Also see:
https://docs.ansible.com/ansible/latest/playbook_guide/playbook_pathing.html#resolving-local-relative-paths

roles/prometheus_node_exporter/meta/main.yaml

@@ -0,0 +1,9 @@
+---
+dependencies:
+  - role: distribution_check
+    vars:
+      distribution_check__distribution_support_spec:
+        - name: Debian
+          major_versions:
+            - "11"
+            - "12"

roles/prometheus_node_exporter/tasks/main.yaml

@@ -0,0 +1,14 @@
+- name: make sure the `prometheus-node-exporter` package is installed
+  ansible.builtin.apt:
+    name: prometheus-node-exporter
+    state: present
+    allow_change_held_packages: true
+    update_cache: true
+  become: true
+
+- name: make sure `prometheus-node-exporter.service` is started and ansibled
+  ansible.builtin.systemd:
+    name: prometheus-node-exporter.service
+    state: started
+    enabled: true
+  become: true