docs: remove mention of FPGA emulators

Remove from both overview and FAQ entries.
Update solution architecture image.

Signed-off-by: Nathan Brown <nathan.brown@arm.com>
Signed-off-by: Yefeng Lin <yefeng.lin@arm.com>
Reviewed-by: Liangxing Wang <liangxing.wang@arm.com>

Change-Id: I48fecfe66914d03e8df3b65a9bb27e2d426cabf6