Refactor parsers

[shreyashankar]

We should put all the data parsers in their own module, and include a page in the documentation (in the API tab). We should mention that users may need to pip install specific packages (e.g., llamaindex) to make user of them.

We should also have a CONTRIBUTING.md that has a section on how to add a new parser.

[shreyashankar]

I wonder if this can be combined with #72