Document MINT

[pat-alt]

@JorgeLuizFranco as long as we're still waiting for #466, we can still already document how gradient-based MINT can be implemented by

1. Adding an SCM to the data.
2. Using any existing gradient-based generator.

So we essentially want an illustrative example that show that using the new functionality, we can generate counterfactuals that make use of the SCM (in line with Karimi's approach). It would be great if you could add this to the documentation, which you can then of course recycle in your final project report/blog post.