docs: move nios2 documentation under Documentation/arch/

docs: move nios2 documentation 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/nios2 into arch/ and fix all in-tree references.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>