Refactor + overview and use cases for Groups-related EE-only features

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

What does this MR do?

• Move Groups doc from /workflow to the correct location user/groups
• Add overview and use cases to groups
• Add overview and use cases to "Lock project membership to group"
• Replace/delete old screenshots

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 => https://gitlab.com/gitlab-org/gitlab-ce/issues/35068

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For #2703 (closed)