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/arm/index.rst

-:orphan:
-
 ================
 ARM Architecture
 ================

Documentation/arm64/index.rst

-:orphan:
-
 ==================
 ARM64 Architecture
 ==================

Documentation/ia64/index.rst

-:orphan:
-
 ==================
 IA-64 Architecture
 ==================

Documentation/index.rst

@@ -117,7 +117,16 @@ implementation.
    :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
 ------------------------

Documentation/m68k/index.rst

-:orphan:
+.. SPDX-License-Identifier: GPL-2.0
 
 =================
 m68k Architecture

Documentation/riscv/index.rst

-:orphan:
-
 ===================
 RISC-V architecture
 ===================

Documentation/s390/index.rst

-:orphan:
-
 =================
 s390 Architecture
 =================

Documentation/sparc/index.rst

-:orphan:
-
 ==================
 Sparc Architecture
 ==================

Documentation/xtensa/index.rst

-:orphan:
+.. SPDX-License-Identifier: GPL-2.0
 
 ===================
 Xtensa Architecture