[Feat] Add custom overlays example to website examples

[tordans]

Target Use Case

The update guide https://visgl.github.io/react-map-gl/docs/upgrade-guide#module-exports references the custom overlays https://github.com/visgl/react-map-gl/tree/7.0-release/examples/custom-overlay.

However, those are not part of the website examples https://visgl.github.io/react-map-gl/examples/; https://visgl.github.io/react-map-gl/examples/custom-overlay returns a 404.

Proposal

Add the custom overlay example to the website.

It looks like the code for that is https://github.com/visgl/react-map-gl/blob/master/website/src/examples-sidebar.js#L24. And it looks like and MDX file like https://github.com/visgl/react-map-gl/blob/master/website/src/examples/custom-cursor.mdx is needed as well.