Make namespace API available to all users
What does this MR do?
This MR makes it possible for a user to query namespaces to which he/she has access. Also, it adds documentation for the existing API.
Why was this MR needed?
Even though the groups API exists, it might still be useful to have an endpoint that tells the namespace type (e.g. user vs. group), especially if a user has access to a number of different projects.