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/board/index.rst b/doc/board/index.rst
new file mode 100644
index 0000000..b47f672
--- /dev/null
+++ b/doc/board/index.rst
@@ -0,0 +1,7 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Board-specific doc
+==================
+
+.. toctree::
+   :maxdepth: 2