docs: add Flat Config example and documentation

[koba04]

This PR adds documentation and an example for Flat Config with ESLint v9.

I've added documentation on configuring this plugin with Flat Config and updated the sandbox to use Flat Config and ESLint v9.

I've never touched plugin implementation and dependencies because this already works with Flat Config and ESLint v9.

I couldn't upgrade ESLint v9 because of issues with eslint-docgen, so I'll work on it at #79 and consider providing a better setup for Flat Config.