Update blackbox likelihood example (#631)

* Update blackbox likelihood example

- Make Op more generic (accepts distinct inputs, returns vectorized loglikelihood), for better transfer learning
- Build Ops more iteratively, and show useful debugging strategies
- Use CustomDist instead of Potential (added a short section at the end showing Potential)
- Actually make use of finite differences as mentioned in the text
- Remove mention of old Cython example and reference new JAX example
- Fix some cross-references (unclear success)

* Address comments from review

Author: ricardoV94