e_tutorials add tutorial on how to built a simplified 1D-2D model with channels

[Jonasvsl]

add tutorial on how to built a simplified 1D-2D model with channels

the example should be a river and not a polder like canal

Not too complex, only focus on how the computational grid works:

tutorial 1D-2D zelf rooster verfijning maken en 1 kanaal toevoegen

[Jonasvsl]

https://docs.staging.3di.live/e_2d_levees_tutorial.html

[Jonasvsl]

the tutorial already available doesn;t contain a river