Steps for reindexing everything with minimal downtime

Guide for re-indexing everything to a new cluster with minimal user impact.

Technically this is "zero downtime" since GitLab keeps working throughout. Depending on how we tackle this the user may have to live with a period of time in which they are either reverting to "Basic search" or alternatively just searching an out of date index.

This is a starting point for gitlab-org/gitlab#210241 (closed) .

This is written in the form of a change request as it will be copied into a change request when we roll this out on GitLab.com . Having it here makes it easier to collaborate on and also we can point our customers to this if they are interested in knowing how to do this themselves.