Improve contributor onboarding document (README/CONTRIBUTING/Dev notes)

[butchtm]

Centralize the placement of the instructions and clarify the steps for onboarding new contributors:
- Forking and cloning the repo (including syncing updates after the fork)
- Setting up a local jupyter notebook dev
- (optionally) Setting up jekyll for local doc site and viewing the updated docs locally
- Making changes to the jupyter notebooks (and updating the code modules)
- Checking that the modifications pass the project requirements (formatting, linting, unit tests, notebook tests)
- Pushing changes back to the repo and making a PR

[joshuacortez]

Addressed by #243