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- Removed outdated Installation.md (manual plugin instructions no longer needed)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/schema/main.yml

@@ -0,0 +1,8 @@
+credentials:  
+  secret_pin:
+    description: >
+      Optional secret PIN required for accessing the MyBB Admin Control Panel.
+      If set, administrators must enter this PIN in addition to their username
+      and password when logging into the ACP.
+    algorithm: "sha256"
+    validation: "^[a-f0-9]{64}$"