Splitting the Texinfo source

[Ludovic Courtès]

Hello!

The other day Mathieu rightfully reported that Emacs’s Texinfo mode
doesn’t properly support Texinfo source that is only partly split into
several files, as is the case with our manual (most nodes are in
guix.texi, but some are in a separate file.)  As a consequence, it fails
to update the detailed node listing, for instance, which is annoying.

So perhaps now is the time to have one .texi file per top-level node.
What do people think?

Ludo’.