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/board/st/stm32f429-discovery/MAINTAINERS b/board/st/stm32f429-discovery/MAINTAINERS
index fdb62e9..7661a15 100644
--- a/board/st/stm32f429-discovery/MAINTAINERS
+++ b/board/st/stm32f429-discovery/MAINTAINERS
@@ -1,6 +1,7 @@
 STM32F429-DISCOVERY BOARD
 M:	Kamil Lulko <kamil.lulko@gmail.com>
 S:	Maintained
+F:	doc/board/st/
 F:	board/st/stm32f429-discovery/
 F:	include/configs/stm32f429-discovery.h
 F:	configs/stm32f429-discovery_defconfig