docs: copy the 'gendocs.sh' script from Texinfo CVS repository

docs: copy the 'gendocs.sh' script from Texinfo CVS repository

As well as the 'gendocs_template' file.  And enhance the "make fetch"
target to keep them synced.  It's better than having to fetch these
files by hand each time a release is to be made.

* Makefile.am (FETCHFILES): List 'gendocs.sh' and 'gendocs_template'.
(fetch): Fetch them, into 'lib/'.
(EXRA_DIST): Distribute them.
* HACKING: Adjust instructions for generation of manuals.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>