documentation: Rework AVA+S32Z

Have a single, more detailed, page containing all the instructions to
run the demo on an AVA and S32Z.

Remove the AVA target from the documentation, as it was an
intermediate step that is not supported anymore.

Issue-Id: SCM-5892
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>
Change-Id: I7c02b9b3897aed3b9cd5f8b38262fd5b980ae733