Follow-up to:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77449

This commit aligns the *bot user for projects/groups* terms for all
project/group access token service accounts within the documentation.
This should clarify the type of service account as well as making the
correlation between bot users for projects and groups better
understandable.

Besides that, this commit also adds a missing chapter about group
access tokens to the token overview page and corrects the linking
within the group access token REST API page.