doc: move doc/board/st/st.rst

'make htmldocs' does not use file doc/board/st/st.rst because the name
matches the directory name. Let's rename it to st-dt.rst.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/board/st/index.rst b/doc/board/st/index.rst
index 8a06a95..9bba42f 100644
--- a/doc/board/st/index.rst
+++ b/doc/board/st/index.rst
@@ -6,5 +6,5 @@
 .. toctree::
    :maxdepth: 2
 
-   st
+   st-dt
    stm32mp1