doc: Refactor into Arm Automotive Solutions

Refactor from Kronos Reference Stack to Arm Automotive Solutions,
but only  when it is referring to generic automotive features, as
opposed to specific features of the Kronos Reference Design.

Amend some sub-headings to make it clearer that the sub-sections are
specific to RD-Kronos.

Refactor the introduction to introduce Arm Auto Solutions prior to
the RD-1 AE Reference Design.

Signed-off-by: Peter Hoyes <peter.hoyes@arm.com>