Document nightly build pipelines/automation

[legoktm]

As a rough summary, we've moved to a model in which nightlies/continuous builds are pushed using GHA from the respective repositories, so:

• sd-client -> sd-apt-test -> droplet
• sd-workstation -> sd-yum-test -> droplet

Then there's also the self-triggering "cleanup" workflows in both repositories.

[legoktm]

The first half of the pipeline of code repo -> LFS repo is added in #183, the latter part of LFS repo -> published on web is not.