Copy the Security dashboard docs to a new location

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

1. Security dashboard (doc/user/security/security_dashboard/index.md) copy and merge content:
   1. https://docs.gitlab.com/ee/user/project/security_dashboard.html
   2. https://docs.gitlab.com/ee/user/group/security_dashboard/index.html
2. Use is_hidden to unlist from the sitemap