[SHIPPING] Enable GitHub Discussions for community Q&A

[wi287g]

GitHub Discussions is friendlier for non-technical contributors (journalists, lawyers, researchers, residents) than Issues, which has a "bug report" connotation.

Tasks

• Repo → Settings → Features → tick Discussions
• Create categories:
  • 📣 Announcements (locked, maintainer-only)
  • 🙋 Q&A (county data questions, source questions)
  • 📊 Data contributions (share new PDFs, open-records responses)
  • 💡 Ideas (feature suggestions from non-technical contributors)
• Pin a welcome post explaining what the dashboard is and how to contribute data
• Link to Discussions from the dashboard's "About" sidebar section