Poliastro: Notebooks don't open in Binder

[DhruvSondhi]

Binder integration of the new modified notebooks(format .mystnb) in index file returns a 404 page not found error whenever accessing the notebooks. Need to investigate in more detail where the problem is arising from. Can be confirmed from #1080 🤔

[astrojuanlu]

Not only the links: the notebooks do not open _as notebooks_. I suspect jupytext is not being properly installed.

[astrojuanlu]

We should also use this opportunity to move requirements.in and requirements.txt to a binder/ directory, like for example https://github.com/mwouts/jupytext/tree/master/binder