Doc: Document shell-ui

[JBWatenbergScality]

Component:

doc

Why this is needed:

We should document shell-ui to explain the user :

• it's role
• how it is deployed

[gdemonet]

IMO, this documentation should be split into two to address different audiences:

• for users, add a UI section to the Operation guide, and put a topic for the common navigation bar in there, with details about how one can add menu entries and change the groups required to access some menus
• for developers of UIs wanting to integrate with our UI, add details about the WebComponent and how to find/use it (OIDC settings, how to handle group-based access control, how to use the theme provider, ...)