infra: indicate that documentation has been superseded

The documentation for infra platforms (except for RD-E1-Edge) has been
superseded by the documentation at
https://gitlab.arm.com/infra-solutions/reference-design/docs/infra-refdesign-docs

.
So, update the readme file accordingly.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I3f616d25e5819823ac2e148dd33eb0acdab5c7c5