Change link from Trixi main page to Trixi2Vtk doc

[DanielDoehring]

The link in the about section links to the Trixi main page
https://trixi-framework.github.io/Trixi.jl/stable/
not the page for visualization with Trixi2Vtk:

https://trixi-framework.github.io/Trixi.jl/stable/visualization/#Trixi2Vtk

Maybe change this?

[sloede]

Would be fine by me!

[ranocha]

Done