Add pigeon package to 'Developing packages & plugins' page

[bryanoltman]

Page URL

https://docs.flutter.dev/development/packages-and-plugins/developing-packages/

Page source

https://github.com/flutter/website/tree/main/src/development/packages-and-plugins/developing-packages.md

Describe the problem

This page does not mention Pigeon (https://pub.dev/packages/pigeon). Pigeon is a Google-developed tool that makes creating plugins much easier, and we should at least mention it here.

Expected fix

https://pub.dev/packages/pigeon is mentioned as a way to develop plugins.

Additional context

No response