Adds documentation for admins to manage users via the user profile
modal for these actions:
- Deactivating a user
- Changing a user's role
- Changing a user's name
Creates two new tab sections because we still want to document
the ability to do these actions through the users section in
the organizational settings modal.
Also cleans up some text in the help center article for changing
a user's role.
Fixes#21318.
Fixes#21415.
This better sets expectatations for the fact that in Zulip, the
Organization settings UI is available read-only to non-administrator
users.
Tweaked by tabbott to update some additional references.