docs: Add 'targets' section

Creates a new section in the docs for target documentation, and adds
some documentation for the stm32f429i-disc1 target.

Change-Id: I0ffbfa56a97458d850834fc0f670b32885b0d2b5
diff --git a/docs/index.rst b/docs/index.rst
index 055690e..169bdcf 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -13,4 +13,5 @@
 
   docs/embedded_cpp_guide
   docs/style_guide
+  targets
   modules