doc: Promote "Channels" as chapter and reorder.

The sectioning becomes:

1.  Specifying Additional Channels     (was 3.)
2.  Using a Custom Guix Channel        (was 2.)
3.  Replicating Guix                   (was 9.)
4.  Channel Authentication             (was 1.)
5.  Primary URL                        (was 7.)
6.  Creating a Channel                 (reworded)
7.  Package Modules in a Sub-directory (was 5.)
8.  Declaring Channel Dependencies     (was 4.)
9.  Specifying Channel Authorizations  (was 6.)
10. Writing Channel News               (was 8.)

* doc/guix.texi (Channels): Move section to chapter.
Reorder the chapter.
Minor tweaks to keep uniformity.
Update the master menu.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

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