Create a repo dedicated to prebuilds and add it to the prebuild guide

From https://github.com/devcontainers/devcontainers.github.io/pull/277#pullrequestreview-1605000577:

> Maybe a separate sample repo just showing off prebuilds would makes it easier for users to get an overview of what is needed. Not a requirement though.

If anyone from the community happens to have one they'd like to contribute to this guide as an example, we'd definitely welcome it!