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.net
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/index.rst b/Documentation/index.rst
index 31f2adc..54ce34f 100644
--- a/Documentation/index.rst
+++ b/Documentation/index.rst
@@ -149,27 +149,11 @@
 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
 -------------------