doc: Milk-V Mars CM and Milk-V Mars CM Lite

Provide a man-page describing the usage of U-Boot on
the Milk-V Mars CM and Milk-V Mars CM Lite boards.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: E Shattow <lucent@gmail.com>
diff --git a/doc/board/starfive/index.rst b/doc/board/starfive/index.rst
index 2762bf7..d369b98 100644
--- a/doc/board/starfive/index.rst
+++ b/doc/board/starfive/index.rst
@@ -6,5 +6,6 @@
 .. toctree::
    :maxdepth: 1
 
-   milk-v_mars.rst
+   milk-v_mars
+   milk-v_mars_cm
    visionfive2