Docs: Add a generic threat model of TF-M

Add a generic threat model document of TF-M.
Create a dedicated folder under docs to contain threat models.

Change-Id: Ifb332d20e1667082d054959bb7d890eda5ce025d
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/docs/index.rst b/docs/index.rst
index 59dfb30..1318d1a 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -89,6 +89,7 @@
     docs/reference/index
     docs/design_documents/index
     platform/ext/index
+    docs/threat_models/index
 
 
 .. toctree::