Update system startup and split document

The system startup document is out of date. Provide a brief updated
description of the Apertis boot process and move to a new "architecture"
section. Split out the boot analysis into its own document and move to a
new "guides" section. Remove the previously performed optimisations as
these describe patched in changes that no longer appear to be a part
of Apertis.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>