Copy the SAST docs to a new location

Move all Secure docs to a new single location. This is about SAST and the MVC is to copy the existing content to the new doc.

1. SAST (doc/user/security/sast/index.md) copy and merge content:
   1. https://docs.gitlab.com/ee/user/project/merge_requests/sast.html
   2. https://docs.gitlab.com/ee/ci/examples/sast.html
2. Use is_hidden to unlist from the sitemap