Document apt repository setup, gotchas and terminology

[legoktm]

Describe the change

Somewhat of a follow-up of my conversation with @creviera, we should document basic Debian repo terminology, how our components are laid out ("nightlies" and then "main" which is actually release candidates that don't match the git main branch), common tasks like how to download an old version and then gotchas, like package deletion not actually working without manual intervention.

[cfm]

I'm having trouble finding it in Google Drive, but @conorsch's "introduction to packaging" presentation might have overview material you could crib as a starting point, including (if I recall correctly) Conor's improvements to https://gist.github.com/cfm/4626264a82d542aa335fd2f2a1336a47.