Create 3.7.2+ migration guide
Created by: slimsag
This change adds a 3.7.2+ migration guide for users upgrading an existing instance from an older version. This applies to anyone upgrading from 3.6 or earlier to the 3.7 or 3.8 series.
We should link to this in the 3.8 blog post, as well, given most users will be going to that version.
Rendered version: https://github.com/sourcegraph/sourcegraph/blob/3.7-migration-guide/doc/admin/migration/3_7.md