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/contributing/code_review_guide.rst b/docs/contributing/code_review_guide.rst
index e9ed969..08d72a28 100644
--- a/docs/contributing/code_review_guide.rst
+++ b/docs/contributing/code_review_guide.rst
@@ -15,7 +15,7 @@
 
 - Read the :doc:`Contributing Process </docs/contributing/contributing_process>`
   to know basic concepts.
-- Read the :doc:`Source Structure </docs/technical_references/source_structure>`
+- Read the :doc:`Source Structure </docs/technical_references/design_docs/source_structure>`
   for structure related reference.
 
 The review guidelines consist of these items: