doc: Add documentation for the Arm vexpress board configs

Create a new documentation section for Arm Ltd boards with a sub-page
for the vexpress board (FVP-A, FVP-R and Juno).

Document how the armv8_switch_to_el1 environment variable can be used
to switch between booting from S-EL2/S-EL1 at runtime on the BASER_FVP.

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