Docs: Move design documents into a dedicated folder

Collect all the design documents under a dedicated design_docs folder.
Update the links in other documents.

Change-Id: I2da761a11317144185e960c539f2245d3d46fd2a
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/docs/integration_guide/os_migration_guide_armv8m.rst b/docs/integration_guide/os_migration_guide_armv8m.rst
index 1ca4e3d..a3b5806 100644
--- a/docs/integration_guide/os_migration_guide_armv8m.rst
+++ b/docs/integration_guide/os_migration_guide_armv8m.rst
@@ -24,7 +24,7 @@
   then it also have to use the
   ``enum tfm_status_e tfm_register_client_id (int32_t ns_client_id)``
   API function provided by TF-M, as described in
-  :doc:`NS client identification documentation </docs/technical_references/tfm_ns_client_identification>`.
+  :doc:`NS client identification documentation </docs/technical_references/design_docs/tfm_ns_client_identification>`.
 - if the OS doesn't support the API mentioned above, it should set
   ``TFM_NS_CLIENT_IDENTIFICATION`` to ``OFF`` in the cmake system.
 - .. Note::