cloud: document manual migrations we're performing

Created by: slimsag

Historically Sourcegraph.com has not been a priority for us, and as such we do not have Sourcegraph.com-only migrations today. See https://github.com/sourcegraph/sourcegraph/issues/12590

This means that today there have been a number of migrations, indexes, etc. that have been applied by hand against the Sourcegraph.com production DB. Some of these have been captured in the Ops incidents log but I fear the vast majority have not.

Recently, we saw another one performed and have a need to perform one that gets us away from this prod-DB-tech-debt issue further.

Until we have a better process in place, this document will serve as a record of which manual migrations were ran against prod, when, why, and how.