docs: create a top-level arch/ directory

As the first step in bringing some order to our architecture-specific
documentation, create a top-level arch/ directory and move arch.rst as its
index.rst file.

There is no change in the rendered docs at this point.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/arch.rst → Documentation/arch/index.rst

@@ -9,20 +9,20 @@ implementation.
 .. toctree::
    :maxdepth: 2
 
-   arc/index
-   arm/index
-   arm64/index
-   ia64/index
-   loongarch/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
+   ../arc/index
+   ../arm/index
+   ../arm64/index
+   ../ia64/index
+   ../loongarch/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

@@ -99,7 +99,7 @@ Architecture-specific documentation
 .. toctree::
    :maxdepth: 2
 
-   arch
+   arch/index
 
 
 Other documentation