Document elasticsearch sizing requirements

What does this MR do?

Documents current requirements for Elasticsearch + GitLab, per a conversation here: gitlab-elasticsearch-indexer#17 (closed)

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• EE specific content should be in the top level /ee folder
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes gitlab-elasticsearch-indexer#17 (closed)