Improve `gen-schema-doc` script dev experience

Right now, executing the `gen-schema-doc` script without setting things up correctly, results in a cryptic error that doesn't tell you that the Python library `json-schema-for-humans` is needed to run the underlying `doc-schema.js` script.

Simply executing `pip install json-schema-for-humans` beforehand fixes this (assuming pip is available). We should either document this somewhere, or make sure that the `doc-schema.js` script gives a more descriptive error when failing.