Cleaning up the documentation

[erikvansebille]

This PR cleans up some of the documentation. Specifically

1. Capitalise Kernels in the text
2. Use AdvectionRK2 as default advection Kernel, as it is faster while the accuracy is comparable for most applications
3. Combine the two Nemo tutorials into one tutorial
4. Separate the Kernels section in the Index into a general Kernels and a Kernels examples section
5. Other minor rewordings