Set up CI to update the model indices

[javagl]

The initial approach for CI in this repository contained a step that was supposed to update the markdown "index" files automatically when a new model is added. When there was a PR, then this CI step tried to update this PR with an update of the index files. Unfortunately, due to some sudden, unannounced, and apparently undocumented change in GitHub actions, this is no longer possible, and it led do #27.

Therefore, the CI currently therefore only checks the validity of the models and folder structure (but does not do any modifications on its own).

It should, however, be possible (and relatively simple) to create a CI step that is triggered when a pull request is merged, and updates the index files. It would need to

• Check out the repository
• Set up PHP
• Run php util/model.php --no-warn --check --update --process-repo --verbose
• Commit the modifications
• Push

Considering that this will only run within this repository, by the GitHub bot that belongs to this repository, there should not be any authorization issues with that.