Create label for architecture to ref

Creating a label so other documentation in the architecture
section can reference this PDP engine documentation.

Issue-ID: POLICY-1163
Change-Id: Ida9caba405e36675f65843b1d49e79175634fa02
Signed-off-by: Pamela Dragosh <pdragosh@research.att.com>
diff --git a/docs/index.rst b/docs/index.rst
index 7e32126..b891ee9 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -1,5 +1,7 @@
 .. This work is licensed under a Creative Commons Attribution 4.0 International License.
 
+.. _apex-doc:
+
 APEX PDP
 ------------------------------------------------
 .. toctree::