doc: boards: add Cubieboard7 documentation

This adds build and flash steps for Actions S700
based Cubieboard7 board.

Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Amit Singh Tomar <amittomer25@gmail.com>
diff --git a/doc/board/index.rst b/doc/board/index.rst
index 51a2ae6..01b233f 100644
--- a/doc/board/index.rst
+++ b/doc/board/index.rst
@@ -6,6 +6,7 @@
 .. toctree::
    :maxdepth: 2
 
+   actions/index
    AndesTech/index
    atmel/index
    coreboot/index