add FPM worker tuning and architecture diagram

Increase PHP-FPM max_children from 5 to 12 to handle concurrent
requests without queuing, sized for a ~3GB VPS. Add Mermaid
architecture diagram to README.

nextcloud/docker-compose.yml

@@ -19,6 +19,7 @@ services:
       - ${DATA_ROOT}/nextcloud/data:/var/www/html/data
       - ./hooks/post-installation.sh:/docker-entrypoint-hooks.d/post-installation/post-installation.sh:ro
       - ./hooks/post-upgrade.sh:/docker-entrypoint-hooks.d/post-upgrade/post-upgrade.sh:ro
+      - ./fpm-tuning.conf:/usr/local/etc/php-fpm.d/zz-tuning.conf:ro
     networks:
       - nextcloud-internal
     logging: