NXDOC: Ensure that people are aware about REST API management endpoint

Customers are usually unaware that a REST API management endpoint exists for nuxeo.

• Searching for it in the doc returns no result
• Documentation does not make it obvious that it can be used as a replacement for the admin center in JSF UI, despite not having a UI for it yet
• It is not clear that more options are actually available than before using this suite of endpoints
• It is not clear what mapping exists between the admin tab in JSF UI and the REST API

Expected: as an administrator, I should be

• aware that an alternative exists to the admin tab in JSF UI
• understand what options are available to me and what can be achieved with them