auto-close PRs

[casperdcl]

There are ~50+~ 80+ PRs mistakenly open here (the getting started guide clearly asks to create a fork; doesn't say open a PR).

• [ ] auto-close new PRs
  • without polluting this repo with e.g. .github/workflows/pr-bot.yml - maybe set up a webhook?
• [x] manually close existing PRs
• [x] with a helpful message saying why and what to do instead: Please fork the repository (first step in https://cml.dev/doc/start/github) to begin experimenting!
• [ ] update the README here and the getting started guide to reduce confusion