docs: Reorganize security pages

Change the page structure and make place for the future threat
assessment section.

Remove references to structured threat models.

Signed-off-by: Gyorgy Szing <gyorgy.szing@arm.com>
Change-Id: Ia8253ceb2b6329ed153626942990595fa666d2fd
diff --git a/docs/security/security-model.rst b/docs/security/security-model.rst
new file mode 100644
index 0000000..12009ca
--- /dev/null
+++ b/docs/security/security-model.rst
@@ -0,0 +1,13 @@
+Security Model
+==============
+
+The security model of Trusted Services builds on the `Platform Security Model`_ v1.1 REL0. For a concept level overview
+please refer to this document.
+
+--------------
+
+.. _`Platform Security Model`: https://developer.arm.com/documentation/den0128/0101b
+
+*Copyright (c) 2020-2025, Arm Limited and Contributors. All rights reserved.*
+
+SPDX-License-Identifier: BSD-3-Clause