doc: Add kas-container documentation for Corstone-1000 FVP

As there is a difference in usage between `kas` and `kas-container` when
building images for the Corstone-1000 FVP, add `kas-container` command
lines to the FVP user guide

This is a minor change to align langdale-dev with main

Signed-off-by: Adam Johnston <adam.johnston@arm.com>