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/add_apt_repository/meta/argument_specs.yaml

@@ -0,0 +1,25 @@
+---
+argument_specs:
+  main:
+    short_description: Add a 3rd party apt repository to the system
+    options:
+      add_apt_repository__https_repo:
+        description: The repository URL uses HTTPS
+        required: true
+        type: bool
+      add_apt_repository__keyring_url:
+        description: URL to the repository's keyring
+        required: false
+        type: str
+      add_apt_repository__keyring_path:
+        description: Path where to store the keyring
+        required: false
+        type: str
+      add_apt_repository__repo:
+        description: The apt source line
+        required: true
+        type: str
+      add_apt_repository__filename:
+        description: Filename in /etc/apt/sources.list.d/
+        required: true
+        type: str

roles/add_apt_repository/tasks/main.yaml

@@ -0,0 +1,33 @@
+---
+- name: Check OS family
+  ansible.builtin.fail:
+    msg: "Can only add apt repositories on Debian-based systems!"
+  when: ansible_facts.os_family != "Debian"
+
+- name: Install required apt packages for adding an apt repository
+  become: true
+  ansible.builtin.apt:
+    name:
+      - ca-certificates
+      - gnupg
+
+- name: Install apt-transport-https if https repository
+  become: true
+  ansible.builtin.apt:
+    name: apt-transport-https
+  when: add_apt_repository__https_repo
+
+- name: Add repository signing key to keychain
+  become: true
+  when: add_apt_repository__keyring_url is defined and add_apt_repository__keyring_path is defined
+  ansible.builtin.apt_key:
+    url: "{{ add_apt_repository__keyring_url }}"
+    keyring: "{{ add_apt_repository__keyring_path }}"
+    state: present
+
+- name: Add repository and update cache
+  become: true
+  ansible.builtin.apt_repository:
+    repo: "{{ add_apt_repository__repo }}"
+    filename: "{{ add_apt_repository__filename }}"
+    update_cache: true