doc: add rst references to distro documentation in stm32mp1 board

Use internal rst reference with :doc: to have a link to distro.rst page
in the generated U-Boot documentation.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
diff --git a/doc/board/st/stm32mp1.rst b/doc/board/st/stm32mp1.rst
index 3f70634..c0b1daa 100644
--- a/doc/board/st/stm32mp1.rst
+++ b/doc/board/st/stm32mp1.rst
@@ -478,7 +478,8 @@
   +-------+--------+---------+------------------------+------------------------+
 
 And the 4th partition (Rootfs) is marked bootable with a file extlinux.conf
-following the Generic Distribution feature (doc/develop/distro.rst for use).
+following the Generic Distribution feature (see :doc:`../../develop/distro` for
+use).
 
 The size of fip or ssbl partition must be enough for the associated binary file,
 4MB and 2MB are default values.