Copy the Dependency scanning docs to a new location

Copy all Secure docs to a new single location. This is about Dependency scanning and the MVC is to copy the existing content to the new doc.

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