doc: Document the graphical installer some more.

* doc/guix.texi (Preparing for Installation): Rewrite to specify the two
installation modes.
(Guided Graphical Installation): New node.
(Manual Installation): New node, with the former sections.
(After System Installation): New node.
* doc/images/installer-network.png, doc/images/installer-partitions.png,
doc/images/installer-resume.png: New files.
* doc/local.mk (dist_infoimage_DATA): Add them.