Add infrastructure that allows to build docs and RELNOTESng without

the need to also create the entire ports tree during a `make release'.
The main motivation behind this is that handling the ports tree still
takes a huge amount of time due to the large number of directories
involved, even on modern disks.

The solution is to establish a list of dependent ports that are
minimally required.  This list needs to be manually maintained in case
the doc ports toolchain changes, and has thus been broken out into a
separate file Makefile.inc.docports.  (release/Makefile has gotten
overly lengthy already anyway.)

Discussed with:	bmah, nik
Reviewed by:	bmah

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