Split Elasticsearch docs to admin and user

What does this MR do?

Split and refactor all Elasticsearch related docs.

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ee/issues/2703
• https://gitlab.com/gitlab-org/gitlab-ee/issues/2667
• Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/2056

Todos

• Add use cases for advanced global search
• Add use cases for advanced syntax search
• Add https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1770#note_28712450 in advanced syntax search
• Mention that we only index HEAD
• Update features.yml gitlab-com/www-gitlab-com!6728 (merged)