Follow-up to https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77236
This commit incorporates two proposals identified during the work
on the group access token API:

- The API docs filename and page title don't match. To have a
  better separation between project and group resources, the file
  was renamed from `resources_` to `project_`, matching the upcoming
  `group_access_tokens.md` page.
- The individual API endpoints now link to the project access token
  main page, allowing quicker access to more infos about the token
  itself.