Update developer documentation

[micahellison]

I wanted to get the ball rolling on updating the contributing docs as per #693. Contributions and suggestions welcome!

• Adding table of contents because it's a little long (plus, some folks just want to jump straight to environment setup details at the end)
• Cleaning up markdown for readability/maintainability (we like those hashes over the hyphens, right?)
• Updating issue links to point to issue template selections instead of generic new issues
• Adding in development notes (I'd like some help here, especially with really clear yet platform-agnostic language)

Checklist

• [n/a] The code change is tested and works locally.
• [n/a] Tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• [n/a] Have you written new tests for your core changes, as applicable?

[wren]

🍰

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.