[docs] restructure 'how to contribute' in developer guide

[jameslamb]

Contributes to #51.

Changes

This PR proposes restructuring the "How to contribute" section in the developer guide, to hopefully make contributing easier for a wider range of possible contributors.

• removes assumption that contributors are using PyCharm
• removes assumption that contributors are on Macs (i.e. no brew install)
• adds code blocks to make instructions copy-pasteable
• replaces reference to "master" branch with "main"
• removes incorrect statement that the CI tests will only run on push if you are in the stitchfix organization

Testing

pre-commit run --all-files

Notes

I've never used virtualenv prior to contributing on this project (I typically use conda/mamba or just Docker instead), so please correct me if I've said anything incorrect about virtualenv or if the pattern I've documented here is different from what you'd like contributors to follow.

Thanks for your time and consideration.

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.
• Reviewers requested with the Reviewers tool ➡️

Testing checklist

Python - local testing

• python 3.6
• python 3.7

[skrawcz]

Looks good. @jameslamb thoughts on my additions?