infra/rdn2cfg1: add user guide for rd-n2-cfg1 platform

Add the initial user guide for the RD-N2-Cfg1 platform software stack.
The user guide points to documents that provide information about
obtaining the platform software stack and using the various features it
supports. A table to list the release tags is included as well.

Signed-off-by: Aditya Angadi <aditya.angadi@arm.com>
Change-Id: I5ae948b420a797e64fe2d0144dcb8e6bbdba89e9