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>
diff --git a/Documentation/index.rst b/Documentation/index.rst
index 864daf8..a322c87 100644
--- a/Documentation/index.rst
+++ b/Documentation/index.rst
@@ -117,7 +117,16 @@
    :maxdepth: 2
 
    sh/index
+   arm/index
+   arm64/index
+   ia64/index
+   m68k/index
+   riscv/index
+   s390/index
+   sh/index
+   sparc/index
    x86/index
+   xtensa/index
 
 Filesystem Documentation
 ------------------------