Add information regarding Elasticsearch and deleted documents

What does this MR do?

Adds a new "Tuning / Deleted documents" section to the Elasticsearch documentation, with some information and possible options regarding deleted documents taking up extra space.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/7611

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [ ] Tests added for this feature/bug
• 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](https://about.gitlab.com/handbook/engineering/quality/test-engineering/).
• 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