docs: Group arch-specific documentation under "CPU Architectures"

To declutter the top-level table of contents (the side bar), this
patch reduces the architecture-specfic documentation to one top-level
item, "CPU Architectures".
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20210312152804.2110703-1-j.neuschaefer@gmx.netSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/arch.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+CPU Architectures
+=================
+
+These books provide programming details about architecture-specific
+implementation.
+
+.. toctree::
+   :maxdepth: 2
+
+   arm/index
+   arm64/index
+   ia64/index
+   m68k/index
+   mips/index
+   nios2/index
+   openrisc/index
+   parisc/index
+   powerpc/index
+   riscv/index
+   s390/index
+   sh/index
+   sparc/index
+   x86/index
+   xtensa/index

Documentation/index.rst

@@ -149,27 +149,11 @@ Architecture-agnostic documentation
 Architecture-specific documentation
 -----------------------------------
 
-These books provide programming details about architecture-specific
-implementation.
-
 .. toctree::
    :maxdepth: 2
 
-   arm/index
-   arm64/index
-   ia64/index
-   m68k/index
-   mips/index
-   nios2/index
-   openrisc/index
-   parisc/index
-   powerpc/index
-   riscv/index
-   s390/index
-   sh/index
-   sparc/index
-   x86/index
-   xtensa/index
+   arch
+
 
 Other documentation
 -------------------