doc: do not install translated man pages if the language was not enabled in LINGUAS
When people cherry-pick the languages they want to install, they certainly
do not care for man pages in other languages. So, instead of always
installing them, the configure script will now automatically detect which
ones need to be installed.
Took opportunity to add a section in the I18N manual to invite contributors
to think about the man pages if contribute to translations.
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>