Document build/upload of upgrade testing boxes

[conorsch]

Description

The project needs documentation for updating the libvirt Vagrant boxes used in the upgrade testing scenario. We have some docs for using the upgrade scenario, but not around box maintenance.

User Research Evidence

Last week after release of 0.11.0, I attempted to upload new boxes, but failed to do so properly, given that the upgrade scenario cannot find 0.11.0 boxes to fetch as of latest develop.

User Stories

As a project maintainer, I want to refer to docs on release-related procedures to ensure I'm following the proper steps.

Misc

Note that we have the outstanding #3832, which should be cleaned up and merged, but it doesn't cover the box creation, merely the use of the already created and uploaded boxes.

[conorsch]

failed to do so properly

I believe what I forgot to do is manually invoke sha256sum on the .box files emitted by make vagrant_packager, then manually update the -metadata.json files with new version strings, URLs, and checksums. If I'd done that prior to running molecule/vagrant_packager/push.yml, I believe we'd be all set now.