Hi Guix,

While looking at how to properly include the documentation
in the makefile in <https://issues.guix.gnu.org/46800>,
I noticed the build recipes for creating $MANUAL.%.texi
and $CHAPTER.%.texi are duplicated (modulo $MANUAL and $CHAPTER).

This patch should make defining new chapters and manuals in the
makefile somewhat simpler and less prone to errors (e.g. avoid
copy-paste + oops I forgot to change the .texinfo and .info file name).

Greetings,
Maxime.