Document access request to project & group
## What does this MR do?
Adds documentation that was missing from !3798.
## 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.
## Does this MR meet the acceptance criteria?
- [ ] Tests
- [ ] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)
/cc @DouweM
See merge request !4685
Achilleas PipinellisandRobert Speicher
Robert Schilling
Jacob Vosmaer