Add a Testing section for manpages in the FDP Primer.

Move the relative information from the Tip section about testing manpages to the new section, along with information about mandoc(1)'s linter, igor, etc. It now covers quite a few cases and common mistakes.

Submitted by: fernape
Reviewed by: debdrup, ygy
Differential Revision: https://reviews.freebsd.org/D27386