Documentation/Docbook/Makefile: process xml files in parallel, based on nproc --all...

Documentation/Docbook/Makefile: process xml files in parallel, based on nproc --all value

Documentation/Docbook/Makefile: process xml files in parallel, based on nproc --all value
This commits introduces a few simple changes, that keep execution of doc compilation serial
( docbook documentation Makefile does not support parallelisation ), but allow parallel
production of xml files. Also compression of man files is now happening in parallel as well.
Number of max threads is limited to nproc --all value.

Orabug: 30079814

Signed-off-by: Alex Burmashev <alexander.burmashev@oracle.com>
Reviewed-by: John Haxby <john.haxby@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>