Feature: Add examples of mixed JS-and-TS linting

Coming from https://github.com/typescript-eslint/typescript-eslint/issues/8777: it'd be nice to show examples of using us in projects that also have `.js`/`.jsx`/etc. files.

* Both using typed linting in JS (i.e. `// @ts-nocheck`) and disabling it
* For typed linting with a `jsconfig.json` and without (just defaulting to the `tsconfig.json`)