doc: create the skeleton to have a documentation for Internationalisation

doc: create the skeleton to have a documentation for Internationalisation

The source of the documentation is in Texinfo format to have the
possibility someday to distribute in more format that just plain text;

There is a new Makefile to generate automatically the plain text file to be
distributed from the source; this doc is also generated by 'autogen.sh'
because a user would expect to find it before compiling the program, so
even before the 'configure' script have been generated, in the case he is
compiling from a git checkout;

There is a new script to generate the documentation, because from the way
we generate it we cannot assume that 'texi2any' is available  and working
on the user's computer; it is also the opportunity to generate a better
looking document (see the description at the beginning of the script);

In this commit, the documentation only contains the base skeleton, it does
not contain yet any useful content because it is worth it's own commit.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>