Add docs on how to contribute to Flutter tooling

[kenzieschmoll]

This could be a top level page here (https://docs.flutter.dev/tools) that provides docs on:

1. Contributing directly to DevTools
2. Writing DevTools extensions
3. VS Code extension promotion

[sfshaza2]

What a great idea! Who would be the SWE collaborator on this?

[kenzieschmoll]

That would be me :) and then @DanTup should review the VS code extension promotion section.

[loic-sharma]

As part of this work, could we migrate the "How to create a Flutter DevTools extension" Medium article to docs.flutter.dev > Tools & Editors > DevTools?

[kenzieschmoll]

We link to this article from the DevTools > DevTools Extensions page: https://docs.flutter.dev/tools/devtools/extensions#build-a-devtools-extension. I don't know if we want to duplicate the content on flutter.dev too. The article is already very close to duplicated on the devtools_extensions README, so adding it to flutter.dev would be a third place where we'd have to keep the docs up to date.