Refactor groups docs

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

Port changes introduced by https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2385 to CE

Update links to new doc (EE-version here with updated links: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2442)

For https://gitlab.com/gitlab-org/gitlab-ce/issues/35068 // https://gitlab.com/gitlab-org/gitlab-ee/issues/2703

Moving docs to a new location?

See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ directory.
• If working on CE, submit an MR to EE with the changes as well.