Update/create documentation

The existing documentation is only provided in the GitHub repository's README.md and is insufficient.

An appropriate and extensive documentation about setting up, configuring and using the MeshUI is required.

* Language configuration
* Base URI configuration
* Keycloak configuraton (once supported)
* General introduction (Admin Area, Edit Area)
* Feature Overview (e.g. Bulk Upload )

Start by reviewing README.md and the current MeshUI screens and configuration paths. Document setup, language and base URI configuration, the planned Keycloak configuration once supported, the Admin Area and Edit Area, and features such as Bulk Upload. Done means the repository contains extensive documentation covering each requested topic.

Written by the indexing model from the issue text.