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/tfm_integration_guide.rst b/docs/integration_guide/tfm_integration_guide.rst
index a543fd0..f0f2a1e 100644
--- a/docs/integration_guide/tfm_integration_guide.rst
+++ b/docs/integration_guide/tfm_integration_guide.rst
@@ -112,7 +112,7 @@
 
 TF-M provides a reference implementation of NS mailbox on multi-core platforms,
 under folder ``interface/src/multi_core``.
-See :doc:`Mailbox design </docs/technical_references/dual-cpu/mailbox_design_on_dual_core_system>`
+See :doc:`Mailbox design </docs/technical_references/design_docs/dual-cpu/mailbox_design_on_dual_core_system>`
 for TF-M multi-core mailbox design.
 
 Interface with non-secure world regression tests
@@ -135,7 +135,7 @@
 NS client Identification
 ========================
 See
-: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>`.
 
 *********************
 Non-secure interrupts