Copilot Usage API Update

[rulasg]

Hi team, the API is scheduled to be released the 23 of April as open beta. Adding more documentation regarding the new API endpoints that will be released with the open beta:

• New data as part of the current usage endpoints
  

• New last activity field added to the existing user management API
  

Originally posted by @Msanzdelrio in https://github.com/githubpartners/Solidify/discussions/317#discussioncomment-9153776