doc: Add architecture specific info to Sphinx TOC tree

Add index.rst for architecture specific info. More docs will be
added later.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/index.rst b/doc/index.rst
index bc2f06a..6d42d94 100644
--- a/doc/index.rst
+++ b/doc/index.rst
@@ -40,6 +40,17 @@
 
    api/index
 
+Architecture-specific doc
+-------------------------
+
+These books provide programming details about architecture-specific
+implementation.
+
+.. toctree::
+   :maxdepth: 2
+
+   arch/index
+
 Indices and tables
 ==================