documentation/Makefile: Add PDF targets.

Addition of various PDF targets allowing the build of books, articles
with the choice of the language. For example, to build both en_US and
fr_FR books, use:
make DOC_LANG="en,fr" pdf-books
Documents are installed in documentation/public/lang/. Targets to clean
the builds are also available.

Reviewed by: ceri, gjb, dbaio
Differential Revision: https://reviews.freebsd.org/D30331