Describe qubes-builder usage as is necessary today.

[ydirson]

This reflects in the doc the consequences of issue #6522, and describes how to build a kernel today. It also gives some hints for #6486 as to what to improve in next generation of qubes-builder.

The kernel build should be linked from main documentation page on the website, but sadly that page is not in this repo.

I flagged this PR as "Draft", as this won't work for possibly a new bug: make qubes-dom0 always rebuilds vmm-xen-dom0, only to fail on core-libvirt-dom0 because of missing sig. So after make sign-dom0 a new make qubes-dom0 run will just regenerate the just-signed rpms with new unsigned ones and fail again.

Also it is not clear to me if we want to unset NO_SIGN by default, which would have an impact on this doc.

[andrewdavidwong]

FYI, you can open a PR as a draft rather than manually adding "WIP" to the title. This is generally less prone to misunderstanding, since it makes it clear to everyone at a glance when you do and don't want your PR to be reviewed and merged.

[andrewdavidwong]

PipelineRetry

[andrewdavidwong]

Just checking in because I haven't seen any communication on this PR in a while. @ydirson, what's the current status from your side?