doc: Copy N1SDP documentation from meta-cassini

The platform documentation will not be built here. It is intended that,
when building the documentation for meta-cassini, the build process will
fetch or otherwise link-to the sources maintained here, rather than
maintaining platform documentation in meta-cassini.

The build changes to meta-cassini will be done later - for now we just
copy the sources here.

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