Docs: Add process for platform removal

Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Change-Id: I3aefb16be610cb60270e8837bd8ce598c25a7f9a
diff --git a/platform/ext/index.rst b/platform/ext/index.rst
index e0d87f3..60422e9 100644
--- a/platform/ext/index.rst
+++ b/platform/ext/index.rst
@@ -7,6 +7,7 @@
 
     readme
     /platform/readme
+    /docs/contributing/platform_deprecation.rst
 
 .. toctree::
     :maxdepth: 4