doc: Add documentation for STM32 MCUs

Add documentation for STM32 MCUs (F4, F7 and H7 series).

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
diff --git a/doc/board/st/index.rst b/doc/board/st/index.rst
index 9bba42f..2a8a4ef 100644
--- a/doc/board/st/index.rst
+++ b/doc/board/st/index.rst
@@ -8,3 +8,4 @@
 
    st-dt
    stm32mp1
+   stm32_MCU