add-ansible-playbook

@torrust

Guide for adding Ansible playbooks to this project. Covers the atomic playbook rule (one playbook = one responsibility), the correct pattern for conditional enablement (in Rust commands/steps, NOT Ansible when: clauses), registering static playbooks in copy_static_templates(), the centralized variables.yml.tera pattern, and a checklist before adding. Use when creating new Ansible playbooks, adding Ansible tasks, or modifying the Ansible infrastructure. Triggers on "ansible playbook", "new playbook", "ansible task", "playbook", "add ansible", "copy_static_templates", or "infrastructure playbook".