Onboarding: Implement `just get-started` and document it in the repo README.md

[ermo]

Motivation

Now that the Rust-based moss and boulder are in production on the infra, it's high time that we offer a decent onboarding experience for people wanting to build serpent-os images and packages from their hosts.

Design considerations

• [x] Build and install moss + boulder to the /usr/local prefix by default with just get-started and list the binaries.
• [x] Enable the ability to override the PREFIX to e.g. /usr or /usr/local
• [x] List link to documentation at the end of the build (to force ourselves to field questions about it and getting it done)

[ermo]

Dumping the requirement for ensuring that boulder is self-documenting ootb (requires merging serpent-os/moss/pull/244) because that PR needs more work. With this, this task is good to go as of 3c7a54bae3e87218d6b2ef9d8e8d11806e92261d