This one has been missing for quite some time (infiniband modules
should have triggered a commit like this back then), finally there
in very crude and draft form for the starters.

This one is a part of a larger rewrite to move away from
distro-centric build-up to configuration-centric one with
the particular packaging being of secondary importance
compared to actual functionality.

Status updates and readability tweaks all over the place.

- introduced generic stage2 subprofile (non-standalone)
- ported installer and rescue over to stage2/{install2,rescue}
- initial stage2/live (needs more work for sure)
- use make-initrd-propagator
- updated and somewhat extended doc/

NB: mind #26133, #26134

reword 'em, structurize 'em, facelift 'em too

...and some more cosmetic fixes

ToC:
- introduced doc/
- features.in/00example/
- READMEs and TODO for more
- code comments clarified

Some unrelated minor fixes bundled either.