docs/BLS: move "boot counting" into the main spec

docs/BLS: move "boot counting" into the main spec

The boot-counting file-renaming entry-sorting part that the boot
loader implements is moved to the main document. The second document
describes a specific implementation that is provided through systemd
units.

The sorting algorithm is extended to say that bad entries should
be sorted later.

I also added a note that bad entries should be available for booting.

For some reason, the second document said that it applies only to EFI systems.
AFAIK there are no implementations for non-EFI, but the specification should
work just fine, if somebody were to implement it. So that part is dropped.

Fixes #23345.

Sadly, bootctl doesn't implement sorting of boot entries with counting :((((
But I'm leaving that for another PR.