Add clear documentation in guides about blocking crucial API endpoints from the outside

[poikilotherm]

Check and highlight for production setups in classic and/or containerized installations:

1. Block /api/admin and /api/builtin-users in Dataverse with a policy
2. Block these endpoints on the Reverse Proxy as well

Provide example snippets for this for Apache and NGINX (classic installations) as well as Traefik (Docker) and Ingress, Gateway API (Kubernetes).

This should be done in the Installation and Containerization Guides.