Use Jupytext to create index for mybinder deployment

[cgranade]

This PR adds a new Markdown file, binder-index.md, that can be used as a landing page for mybinder.org deployment. By adding jupytext as a Dockerfile dependency, binder-index.md appears to Jupyter Notebook as a Q#-language notebook, enabling some really neat experiences. For example, if you follow the "CHSH Game" link below, that takes you to a Jupyter Notebook rendering of that sample's README.md:

That README.md is then itself an executable PowerShell-language notebook, such that instructions in the ordinary README can be executed in-place:

So far, this PR has been tested in VS Code devcontainers, but once merged in we can test with binder before redirecting the ★binder branch to point to the new samples landing experience.

[guenp]

@cgranade This is indeed really neat! would it be an idea to add a note on how to use said features to CONTRIBUTING.md?

[cgranade]

@cgranade This is indeed really neat! would it be an idea to add a note on how to use said features to CONTRIBUTING.md?

That's a good idea, @guenp, totally agreed. I think that should be deferred until after we test with mybinder deployment, however, so that we can make sure any advice that we add there is as accurate as possible.

[rmshaffer]

Looks really cool; I look forward to trying this out!

[msoeken]

This is a great overview page. Thanks!