[META] AI policy (#7914)

* Add `CONTRIBUTING.md` and link it in the readme

* Add TBD PR template

* Merge `DEVELOPMENT.md` -> `CONTRIBUTING.md`

* Revise `PULL_REQUEST_TEMPLATE.md`

* Have checkboxes

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,10 @@
+<!--
+Thanks for your contribution!
+-->
+
+- [ ] Closes #xxxx <!-- Replace xxxx with the GitHub issue number -->
+- [ ] This PR follows our [AI policy](https://github.com/RustPython/.github/blob/main/AI_POLICY.md)
+
+## Summary
+<!-- What's the purpose of the change? What does it do, and why? -->
+