update Contributing doc and PR Template

[yasskin]

PR Checklist

Storybook can be viewed here

• Manually tested across supported browsers

  • [x] Chrome
  • [ ] Firefox
  • [x] Safari

• [ ] Unit tests written (common at minimum)

• [x] PR has one of the semver- labels

• [ ] Two core team engineer approvals

• [ ] One core team UX approval

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