Update LDAP guide

`docs/how-to/enable-ldap.md`

This guide suggests that all apps can be deployed in the same model on Kubernetes, but it is better practice to keep GLAuth in a separate model.

The page must be updated to account for cross-model relations on K8s, which will require minor restructuring and reformatting of some tab elements.

Start with docs/how-to/enable-ldap.md and review how the guide currently models app deployment on Kubernetes. Update it to account for GLAuth in a separate model and cross-model relations, then verify that the tab elements remain coherent after the restructuring and reformatting.

Written by the indexing model from the issue text.