docs: move mips under arch

and fix all in-tree references.

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.

Signed-off-by: Costa Shulyupin <costa.shul@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230725043835.2249678-1-costa.shul@redhat.com