docs: add arch doc directories to the index

Now that several arch documents were converted to ReST,
add their indexes to Documentation/index.rst and remove the
:orphan:  from them.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/sparc/index.rst

@@ -1,5 +1,3 @@
-:orphan:
-
 ==================
 Sparc Architecture
 ==================