Add license

[erlend-sh]

• Adds the PolyForm NonCommercial license
• Adds first draft of our 'Contributor License Compromise'

it should be accompanied by

• a PR template that has a simple [ ] checkbox for contributors to acknowledge their awareness and compliance with our CLC
• a github action that ensures the checkbox has been checked in PRs made by non-team members.

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For precedence, see for example CrabNebula's devtools.

[zicklag]

I hooked up this bot to ask users to make a comment saying that they have read and agree to the doc:

https://github.com/contributor-assistant/github-action?tab=readme-ov-file#demo-for-step-1

It records signatures in a JSON file in a different branch of the repo, and doesn't ask them if they've already signed.

Seems good 'cause then we have a history of signatures, but don't require redirecting to a CLA signing site or anything like that, they just have to copy and paste a comment.