[slides][dunemodule] add slide set to index.md

slides/index.md

@@ -4,6 +4,7 @@ title: DuMuX Course Slides
 - [Introduction to DuMu^x^](./intro.html)
 - [Runtime Parameters and Grids](./runtimeparams-grids.html)
 - [Property System](./properties.html)
+- [New Dune Module](./dunemodule.html)
 - [Model](./model.html)
 - [Introduction to Multidomain](./multidomain.html)
 - [Discrete Fracture Modeling](./fractures.html)