docs: reworder/rewrite BLS to read more like a specification

docs: reworder/rewrite BLS to read more like a specification

I tried not to introduce any semantic changes, but to reorder the whole
text to be more usable as a reference specification: more sections are
created and the discussion and justifications are moved to the end.
Also, "BIOS" is changed to "firmware" in various places, and other parts
of the text that made sense when this was originally written are now dated
are adjusted. I separated and extended the examples a bit.

The abstract at the top ("TL;DR: Currently there’s no common boot scheme…")
is dropped. It didn't seem to fit anywhere.