Add docs page on migrating and backup
Created by: dadlerj
See a rendered preview at https://github.com/sourcegraph/sourcegraph/blob/e905d52f643d0f2bc2b02295855aba5d74df7a81/doc/admin/install/migrate-backup.md
This page ultimately needs much more detail on how to dump/load Postgres data for each deployment type (CC @sourcegraph/distribution ). Currently, I only link out to https://docs.sourcegraph.com/admin/install/docker-compose/migrate which only covers one case on each side.