infra/rdn2: add user-guide.rst for rdn2 platform

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

Signed-off-by: Aditya Angadi <aditya.angadi@arm.com>
Change-Id: I441744e6a8486fd5bdc30ba3d8b8c38eb6be1022