refactor(docs): add a sub-menu for ST platforms

In order to ease introduction of new STM32 MPUs platforms, a dedicated
ST sub-menu (and directory) is created.
The old page is kept, but with an orphan parameter to avoid build
issues with the docs, and to avoid listing it in the menu. It is
updated to just have links with the new pages.
A new page STM32 MPUs is created to group common options for all STM32
MPUs.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: I799b57967d76a985835c7a3d9d6ab21beb44ba40
diff --git a/docs/plat/index.rst b/docs/plat/index.rst
index 2eec68c..7a05fb6 100644
--- a/docs/plat/index.rst
+++ b/docs/plat/index.rst
@@ -42,7 +42,7 @@
    rockchip
    socionext-uniphier
    synquacer
-   stm32mp1
+   st/index
    ti-k3
    xilinx-versal-net
    xilinx-versal