n1sdp: update documentation for 2021.10.12 platform release

A new user guide is prepared to provide instructions for bash script
based build setup, the steps to build the firmware, minimal BusyBox
and the Ubuntu distribution with new scripts are captured.

Other documents are updated as applicable for the 2021.10.12 release.

Change-Id: Id103cde03dce2de6b5359b8637fda7a31deced76
Signed-off-by: Khasim Syed Mohammed <khasim.mohammed@arm.com>