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 <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/arch/index.rst

···
 .. toctree::
    :maxdepth: 2
 
-   ../arc/index
+   arc/index
    ../arm/index
    ../arm64/index
    ../ia64/index

MAINTAINERS

···
 L:	[e-mail]
 S:	Supported
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/vgupta/arc.git
-F:	Documentation/arc/
+F:	Documentation/arch/arc
 F:	Documentation/devicetree/bindings/arc/*
 F:	Documentation/devicetree/bindings/interrupt-controller/snps,arc*
 F:	arch/arc/