[fdp-primer][manpages] Add Testing section

[fernape]

Summary

I think we can add a Testing section for manpages. I moved the information from
the "Tip" about testing pages to a new section along with information about
mandoc(1)'s linter, igor, etc. I think the information there covers quite a few
cases and common mistakes.

Test Plan

• igor seems happy
• spell checking looks good
• the html page is rendered properly

[debdrup]

Would it make sense to mention hunspell/aspell too? While igor does do some spellchecking, it does so on a rather limited set of words defined in the init_spellingerrors function.

[fernape]

In D27386#611787, @debdrup wrote:

Would it make sense to mention hunspell/aspell too? While igor does do some spellchecking, it does so on a rather limited set of words defined in the init_spellingerrors function.

Sure!

I will come up with something soon. Thanks for the hint!

[debdrup]

How about something like:

<para>Spell-checking with fully-featured dictionaries is encouraged, and can
  be accomplished by using <package>textproc/hunspell</package> or
  <package>textproc/aspell</package> combined with
  <package>textproc/en-hunspell</package> or
  <package>textproc/en-aspell</package>, respectively.</para>

[fernape]

Include reference to spell checkers.

Suggested by debdrup@

[ygy]

Looks good to me!

[debdrup]

Looks good to me, too. :)

[fernape]

Thanks for the review!