managed: machine upgrade guide
Created by: bobheadxi
Creates a guide, partially leveraging the existing tooling, for upgrading the machine for a managed instance.
Closes https://github.com/sourcegraph/customer/issues/481 , follow-up to https://github.com/sourcegraph/deploy-sourcegraph-managed/pull/96#issuecomment-915613916 , required to resolve https://github.com/sourcegraph/customer/issues/478
The first commit isn't worth reviewing (just misc. readability and structure changes) - the bulk of the changes is in https://github.com/sourcegraph/about/commit/882cd70baa7b59b6fae426d1c132a52e3b219f43