Would like expanded releng docs for building OS images

If you're getting started from the perspective of I normally build an OS image, but not in Omicron context, and have an urgent need to turn that into one that does, I think expanding the docs to cover some of the expected pre-reqs and which parts of the other Omicron dev docs are required might be useful. From reading the current docs one might assume that it'd be reasonable to just clone omicron and run cargo xtask releng --helios-local --helios-dir ~/src/path/to/helios. There are a few additional things I've found that need to be set up, which I expect are true regardless of what one is doing:

• Creating required ZFS datasets
• Having downloaded dendrite bits (cargo xtask download dendrite-openapi)

From the --help and current docs it's not clear if I should have already built the illumos bits (I did) or not. I'm assuming it wants me to at this point in the process, but I'm also not sure how it would pick debug vs. non-debug.

Start with the current releng documentation and run cargo xtask releng --help to compare the documented flow with the reported prerequisites. Check how required ZFS datasets, cargo xtask download dendrite-openapi, and existing illumos debug or non-debug builds fit into the process. Done means the docs clearly list setup steps and explain which build artifacts each mode expects.