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.

docker/wordpress/Dockerfile

@@ -0,0 +1,13 @@
+FROM wordpress:6.9-php8.2
+
+# Install handy tooling (git, mariadb client, wp-cli) for local development.
+RUN apt-get update \
+	&& apt-get install -y --no-install-recommends git less vim unzip mariadb-client \
+	&& rm -rf /var/lib/apt/lists/* \
+	&& curl -o /usr/local/bin/wp https://raw.githubusercontent.com/wp-cli/builds/gh-pages/phar/wp-cli.phar \
+	&& chmod +x /usr/local/bin/wp
+
+WORKDIR /var/www/html
+
+# Match the default Linux UID/GID to avoid permission headaches with bind mounts.
+RUN usermod -u 1000 www-data && groupmod -g 1000 www-data