WIP: Documentation membership refactor

Open Achilleas Pipinellis requested to merge docs/membership-refactor into master

What does this MR do?

Refactor the membership docs.

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.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/22934.