[doc] Differentiate between test, tutorial/documented, tutorial/undocumented

Currently we have two general "getting started tutorials", however many of our tests are also thought of tutorials as they demonstrate a certain model.

I suggest to distinguish between tests in the test -folder that are specifically designed to test certain features and are small/run rather fast, and tutorials in the tutorial-folder that are designed to show a certain feature, e.g. how to use the tensor grid factory in a Richards model to refine towards the soil top. I further suggest to create two new subfolders documented and undocumented where all documented tutorials have a detailed latex/markdown problem and feature description. In the future, a number of maybe 10 such documented tutorials would be nice. All tutorials in undocumented can potentially become documented in the future.

Of course tutorials should also tested by the automated build system.