Document how to use AI rules

### Use case

We are currently working on integrating AI agents to work with project.
Ideally the presets should be as close to Flutter conventions as possible.

I saw here are rules defined to be integrated with different models:
https://github.com/flutter/flutter/tree/master/docs/rules

But it isn't documented what these files actually are representing and how they actually can be used. They are not referenced anywhere (in the flutter project).

I was also confused whether these app rules are meant for app developers or for the Flutter project itself.

### Proposal

- Explain for which purpose the rules are NOT made for (Flutter repository itself, Flutter packages (?)) or additionally I'd propose the title to be changed to `AI rules for Flutter Apps`
- Add a reference or / and explain more detailed on how to include AI rule files in a git / Flutter project (cherry on top: add an example project)
- Reference per model the first instructions or make it more clear that the limit docs also include the instructions for setup