Update Elasticsearch docs and add "projects_not_indexed" rake task

What does this MR do?

This MR does two things:

1. Improves on existing ElasticSearch documentation
2. Adds a rake task to detail unindexed projects

What are the relevant issue numbers?

This is stemming from an ElasticSearch deep dive Support did: gitlab-com/support/support-team-meta#1458 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

Closes #10195 (closed)