doc: Add board specific info to Sphinx TOC tree

Add index.rst for board. 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 6d42d94..9ae2e16 100644
--- a/doc/index.rst
+++ b/doc/index.rst
@@ -51,6 +51,17 @@
 
    arch/index
 
+Board-specific doc
+------------------
+
+These books provide details about board-specific information. They are
+organized in a vendor subdirectory.
+
+.. toctree::
+   :maxdepth: 2
+
+   board/index
+
 Indices and tables
 ==================