refactor(web-app-mybb): remove obsolete Installation.md, introduce schema for secret_pin, and rework task/vars handling

- Removed outdated Installation.md (manual plugin instructions no longer needed)
- Added schema/main.yml with validation for secret_pin
- Added config.php.j2 template to manage DB + admin config
- Refactored tasks/main.yml to deploy config.php instead of legacy docker-compose
- Removed setup-domain.yml (TLS/domain handling moved to core roles)
- Updated docker-compose.yml.j2 to mount config.php and use new vars
- Cleaned up vars/main.yml: standardized MYBB_* variable names, added MYBB_SECRET_PIN, config paths, and container port

See ChatGPT conversation: https://chatgpt.com/share/68b0ae26-93ec-800f-8785-0da7c9303090

roles/web-app-mybb/templates/config.php.j2

@@ -0,0 +1,13 @@
+<?php
+$config = array();
+$config['database']['type'] = 'mysqli';
+$config['database']['database'] = '{{ database_name }}';
+$config['database']['table_prefix'] = 'mybb_';
+$config['database']['hostname'] = '{{ database_host }}';
+$config['database']['port'] = '{{ database_port }}';
+$config['database']['username'] = '{{ database_username }}';
+$config['database']['password'] = '{{ database_password }}';
+$config['database']['encoding'] = 'utf8mb4';
+$config['super_admins'] = '{{ MYBB_ADMIN_USER_ID }}';
+$config['secret_pin'] = '{{ MYBB_SECRET_PIN  }}';
+$config['admin_dir'] = 'admin';