Overhaul contribution guide & add a AI policy

CONTRIBUTING.md

@@ -1,11 +1,16 @@
 Contributing to pretix
 ======================
 
-Hey there and welcome to pretix!
+Welcome to pretix, we are happy that you would like to contribute.
+Before you do so, please make sure to read the following documents:
 
-* We've got a contributors guide in [our documentation](https://docs.pretix.eu/dev/development/contribution/) together with notes on the [development setup](https://docs.pretix.eu/dev/development/setup.html).
+- [Contribution workflow](https://docs.pretix.eu/dev/development/general.html)
+- [AI-assisted contribution policy](https://docs.pretix.eu/dev/development/ai.html)
+- [Coding style and quality](https://docs.pretix.eu/dev/development/style.html)
+- [Development setup](https://docs.pretix.eu/dev/development/setup.html)
+- [Code of Conduct](https://docs.pretix.eu/dev/development/codeofconduct.html)
 
-* Please note that we have a [Code of Conduct](https://docs.pretix.eu/dev/development/contribution/codeofconduct.html) in place that applies to all project contributions, including issues, pull requests, etc.
-
-* Before we can accept a PR from you we'll need you to sign [our CLA](https://pretix.eu/about/en/cla). You can find more information about the how and why in our [License FAQ](https://docs.pretix.eu/trust/licensing/faq/) and in our [license change blog post](https://pretix.eu/about/en/blog/20210412-license/).
+Before we can accept your first PR we'll need you to sign [our **Contributor License Agreement** (CLA)](https://pretix.eu/about/en/cla).
+You can find more information about the how and why in our [License FAQ](https://docs.pretix.eu/trust/licensing/faq/) and in our [license change blog post](https://pretix.eu/about/en/blog/20210412-license/).
 
+**Before contributing new functionality, always open a discussion first.**