documentation/Makefile: Add HTML targets for offline use

Addition of various HTML 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" html-books
Documents are installed in documentation/public/lang/. Targets to clean
the builds are also available.
The produced files do not include some metadatas, they are intended for
offline use.
Reviewed by: ceri
Differential Revision: https://reviews.freebsd.org/D30718