feat: Implement Siti Stock Plugin for WooCommerce integration

- Added core plugin structure with main class Siti_Stock_Plugin.
- Implemented settings management through Siti_Stock_Settings.
- Developed admin interface for settings configuration via Siti_Stock_Admin.
- Created inventory management with external stock handling in Siti_Stock_Inventory_Manager.
- Integrated synchronization service to fetch and apply stock updates from external API in Siti_Stock_Sync_Service.
- Added custom product data store to manage combined stock values in Siti_Stock_Product_Data_Store.
- Registered hooks for admin menus, settings, and synchronization processes.
- Implemented REST API endpoint for triggering stock sync.
- Added cron scheduling for automatic stock synchronization.
- Included localization support for Dutch language.

.gitignore

@@ -0,0 +1,20 @@
+# OS cruft
+.DS_Store
+Thumbs.db
+
+# IDE / tooling
+.idea/
+.vscode/
+*.code-workspace
+
+# Dependencies & builds
+node_modules/
+vendor/
+dist/
+
+# Logs / env
+*.log
+.env*
+
+# Docker artifacts
+docker/wordpress/wp-content/