docs: import Hafnium SPMC design doc from TF-A

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I6a74fc28cc79623589d0cbd1da1bacc086188a40
diff --git a/docs/design/index.rst b/docs/design/index.rst
new file mode 100644
index 0000000..5e8ae46
--- /dev/null
+++ b/docs/design/index.rst
@@ -0,0 +1,9 @@
+Design
+======
+
+.. toctree::
+   :maxdepth: 1
+   :caption: Contents
+
+   secure-partition-manager
+   ffa-manifest-binding