docs: Move arc architecture docs under Documentation/arch/

docs: Move arc architecture docs under Documentation/arch/

Architecture-specific documentation is being moved into Documentation/arch/
as a way of cleaning up the top-level documentation directory and making
the docs hierarchy more closely match the source hierarchy.  Move
Documentation/arc into arch/ and fix all in-tree references.

Acked-by: Vineet Gupta <vgupta@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>