Use master branch of tutorials

[uekerman]

Currently, we render the content of the develop branch of the tutorials.

This is good because:

• We encourage users to contribute, and then it is nice if you can directly see your edits live after the PR is merged.
• We don't forget to merge these changes back and forth.

This is bad because:

• The link "get the case files of this tutorial" goes to master to access released versions. If someone now adds a new solver, this information is the README, but the case files are not accessible. If someone adds a new case, we either have a dead link or we link to develop and forget to update later.

A proper solution would probably be to render the master branch or the latest release and combine this with some automation to merge fixes back to develop. And release the tutorials more often (independently of the distribution).