Docs: Refactor Elasticsearch docs

Problem to solve

Over time, the Elasticsearch docs have grown to more than 1,300 lines. We should look for ways to break up the page.

Proposal

The new structure could look like this:

  • Elasticsearch installation
  • Elasticsearch indexing
  • Enable advanced search
  • Zero-downtime reindexing
  • Advanced search migrations
  • Advanced search Rake tasks
  • Optimize advanced search settings
Edited by Ashraf Khamis