Add zero-downtime re-indexing documentation

What does this MR do?

This MR adds documentation about using ElasticSearch alias to enable zero-downtime re-indexing.

In this process, we are leveraging the ElasticSearch re-index API and index alias such as we decouple the index used by GitLab from the index we use for reindexing.

This method enables us to use an atomic process to swap the freshly indexed index online, without any downtime.

Conformity

• [-] Changelog entry
• Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Closes #213918 (closed)