add one page for reference design

Signed-off-by: Chi-Sheng (Daniel) Shih <[email protected]>

docs/index.md

@@ -18,6 +18,7 @@ For detailed documents of Autoware Universe components, see [Autoware Universe D
 - [Tutorials](tutorials) pages explain several tutorials that you should try after installation.
 - [How-to guides](how-to-guides) pages explain advanced topics that you should read after you get used to Autoware.
 - [Design](design) pages explain the design concept of Autoware.
+- [Reference Design](reference-design) pages explain the reference design of Autoware-based autonomous vehicles.
 - [Contributing](contributing) pages explain how to contribute to Autoware.
 - [Datasets](datasets) pages contain information about datasets that can be used with Autoware.
 - [Support](support) pages explain several support resources.