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!

Start by locating the existing prebuild guide in this repository and reviewing the linked pull request context. Create a dedicated prebuild sample repository, then add it to the guide as an example; done means the repository exists and the guide links to it.

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