manual/user guide/customization: move section on storing configuration

This patch moves the section 'Basics for storing the configuration' to a
separate file and upwards in the manual section order, to line up with the
listing of customization actions in the introduction of the chapter. The
text itself is currently unchanged, this will be handled in the next patch.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>