docs: separate the GENDOC infra from our manual definition

Move the GENDOC infra to its own file, so it is even less tied to our
manual document, so that it is more obvious that GENDOC is an infra like
our packages infras, and 'manual' is a document like we have packages.

Ideally, this new file should better go in docs/ rather than in package/ .
However, docs/ is already full of our website stuff, so adding it in
there would just serve to clutter the website.

So, let's just put alongside the other infrastructures, in package/ .

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>