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

Cc: "David S. Miller" <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/arch/index.rst

···
    ../riscv/index
    ../s390/index
    ../sh/index
-   ../sparc/index
+   sparc/index
    x86/index
    xtensa/index

drivers/sbus/char/oradax.c

···
  * the recommended way for applications to use the coprocessor, and
  * the driver interface is not intended for general use.
  *
- * See Documentation/sparc/oradax/oracle-dax.rst for more details.
+ * See Documentation/arch/sparc/oradax/oracle-dax.rst for more details.
  */
 
 #include <linux/uaccess.h>