Manual structure

[Andreas Enge <andreas@enge.fr>]

Hello,

I would like to suggest moving some chapters and sections around in the
manual. For keeping this short, I will refer to the section numbers.
My main motivation are the four digits now denoting subsections in 7;
this is an indication that we are putting too much into this chapter.
Notice that this chapter, "GNU Distribution", has been introduced in the
beginning to speak about the packages of Guix; now it is more concerned with
GuixSD. Loosely speaking, I would like to strip off the highest level of 7
and make subchapters of 7.1, 7.2 and so on.

Moreover, I see the manual as having three big parts:
- introduction (chapter 1, well, so is is rather small)
- Guix (chapters 2-6 and 7.3-7.8)
- GuixSD (chapters 7.1 and 7.2)
I did not think about 8 and where it belongs.

Concretely, I suggest the following:
- Rename "7.6 Packaging Guidelines" to "Packages", and move it one level up
  between the current 6 and 7 as the new chapter 7. Somehow integrate 7.3 
  to 7.8 here.
- Move 7.1 and 7.2 to new chapters 8 and 9.

What do you think?

Changing the layout is made more difficult by the fact that all menus
are hand-coded; do I remember wrongly that this can be done automatically
in texinfo? If yes, it would be easier to make such a change first.

Andreas