infra: introduce location for release collateral

For all subsequent releases, the release collateral will be located at
docs/infra/releases. A directory for each release will be created under
this path and the release collateral placed there.

Add the release note for RD-INFRA-2021.09.17 release and migrate the
change log notes for RD-INFRA-2021.08.11 release into this release
collateral location.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I4b5a0edf256f6b15df218c1b5bb145736c4bd07a