Document access request to project & group

What does this MR do?

Adds documentation that was missing from !3798 (merged).

Are there points in the code the reviewer needs to double check?

I've taken the opportunity to update the images in the "Project users" documentation page, I can remove the commit if it's not welcome!

What are the relevant issue numbers?

Closes #18584 (closed).

Does this MR meet the acceptance criteria?

• Tests
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

/cc @DouweM