Update the 12.0R release documentation following the recent move
from the base repository with changes provided by hrs@, and further
tweaks from myself.

Of note:

• Consolidate duplicated logic throughout the Makefiles and shared logic, and move files to more sane locations where they may be shared by other parts of the build tools.
• Update location of several entity files.
• In the top-level Makefile, only include the hardware page if the MAN4PAGES knob is set. This is done to prevent build failures on the website until the webupdate script and wrapper are updated, which will require some more thought. [1]

This fixes the 'reldoc' target in release/Makefile in head, found
to be broken a few days ago.

[1] Changes from myself.

Patch provided by: hrs
Sponsored by: The FreeBSD Foundation