Tutorials overview and visualization should be in the tutorials repo

The following pages still require separate PRs to this repository, but could be moved to the tutorials repository:

- https://precice.org/tutorials.html
- https://precice.org/tutorials-visualization.html

There is no reason to keep them here, and they easily get outdated.

Start by locating the files that generate tutorials.html and tutorials-visualization.html, then compare their current links and navigation with the tutorials repository. Move the two pages there and remove their separate maintenance requirement from this repository; done means both pages remain accessible and no longer require PRs here.

Written by the indexing model from the issue text.