books/fdp-primer: Fix outdated information

[dbaio]

Summary

This is not a rewrite, only removing outdated or wrong information and
adding small improvements, although chapters 'Translations' and 'PO
Translations' will need a full rewrite.

Test Plan

The resulting document can be seen
here.

[dbaio]

• Remove wrong PO file name information

[crees]

Remove doc-committers in favour of voluntary docs reviewers.

[crees]

Docs changes are fine, if doceng is happy too.

[bcr]

Looks good to me as well. If no one else from doceng has any objections, this can be committed.
Thanks for the work!

[carlavilla]

Please, don’t commit this change.
I’m working right now with the FDP, I’ll commit this as I talked with @dbaio in the freebsd-doc mailing list