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::
diff --git a/docs/integration_guide/services/tfm_ps_integration_guide.rst b/docs/integration_guide/services/tfm_ps_integration_guide.rst
index 2c6da7b..50c83a4 100644
--- a/docs/integration_guide/services/tfm_ps_integration_guide.rst
+++ b/docs/integration_guide/services/tfm_ps_integration_guide.rst
@@ -293,7 +293,7 @@
 processing environment. It provides a dedicated API to retrieve the client ID
 which performs the service request.
 
-: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>`
 provides further details on how client identification works.
 
 PS service uses that TF-M core API to retrieve the client ID and associate it
diff --git a/docs/integration_guide/services/tfm_psa_proxy_integration_guide.rst b/docs/integration_guide/services/tfm_psa_proxy_integration_guide.rst
index 9b8ed3d..4fb467f 100644
--- a/docs/integration_guide/services/tfm_psa_proxy_integration_guide.rst
+++ b/docs/integration_guide/services/tfm_psa_proxy_integration_guide.rst
@@ -9,7 +9,7 @@
 to a Secure Enclave, this way virtually providing all the PSA RoT services.
 Proxy can only be used in IPC model, for context and design details please
 check the
-:doc:`Secure Enclave design document </docs/technical_references/secure_enclave_solution>`.
+:doc:`Secure Enclave design document </docs/technical_references/design_docs/secure_enclave_solution>`.
 
 Currently to forward the PSA Client call parameters Proxy must read them with
 ``psa_read`` into a memory area shared with the Secure Enclave. (Similarily
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