Docs: Fix broken PDF version of the User guide

- Limit FWU design document content table depth to comply with Latex
- Reduce the Design Documents content table to titles only

Signed-off-by: Anton Komlev <anton.komlev@arm.com>
Change-Id: Ia9df1b2a896f7a662bdd84e671fc091a227e25e6
diff --git a/docs/technical_references/design_docs/index.rst b/docs/technical_references/design_docs/index.rst
index bdd2d03..e091bbb 100644
--- a/docs/technical_references/design_docs/index.rst
+++ b/docs/technical_references/design_docs/index.rst
@@ -2,7 +2,7 @@
 ================
 
 .. toctree::
-    :maxdepth: 2
+    :maxdepth: 1
     :glob:
 
     */index