Docs: add indexes for monitoring and performance monitoring

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

What does this MR do?

Ports https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15942 to EE

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.