Add information to "Contributing"

[lilyminium]

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Is your feature request related to a problem? Please describe.

• Reviewing notebooks with git diffing is difficult
• Jupyter does not automatically create kernels for conda or pip environments
• Adding references is tedious

Describe the solution you'd like

• [x] Add the "make github" snippet to Makefile and instructions + explanation + link to gh-pages to user guide
• [ ] Add pull request template asking for link to built docs
• [ ] Add instructions for making a new Jupyter environment
• [ ] Add explanation and instructions for how to do references