sys/*/conf/*, docs: fix links to handbook
While here, fix all links to older en_US.ISO8859-1 documentation
in the src/ tree.
PR: 255026
Reported by: Michael Büker <freebsd@michael-bueker.de>
Reviewed by: dbaio
Differential Revision: https://reviews.freebsd.org/D30265
(cherry picked from commit c1a148873d6488f1dac1cfb207a5927cef89888e)