Add "launch" link and "download ipynb" functionality for myst notebooks

[choldgraf]

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Currently we have two features that make a hard assumption about the input file extension being ipynb:

IPYNB downloading:

and Launch Links:

We should add in some logic to check whether the input file is a MyST notebook file, and add these in for those.

For the launch links, we will also need to add documentation to users to tell them how they can configure their repositories to open MyST notebooks with jupytext automatically at launch so they show up as proper notebooks

[jstac]

I would have thought that we always want to offer ipynb files for download, in their executed form, and not bother offering md files at all.

For your average reader (e.g., student in a data 8 course), a myst file will mean little to them. But an executed ipynb will be valuable.

Those who want a myst file will probably be relatively tech-savvy and can convert via Jupytext.

The other related point is that @gregcaporaso and others point out that we have links to source so that readers can easily open PRs. In which case they have access to the myst files anyway.

[choldgraf]

sure thing - it's just that the actual raw source files are already downloadable so if we wanted to only let people download ipynb files it would actually mean we need to remove some functionality. Is this what you're suggesting?

Alternatively, we could only offer ipynb download for the MyST notebooks, but not for "regular" myst markdown documents.

[jstac]

if we wanted to only let people download ipynb files it would actually mean we need to remove some functionality. Is this what you're suggesting?

I'm just saying that, IMO, we should emphasize ipynb downloads over md downloads. So, in the pic at the top of this issue, you have "md" and "pdf" as the two download options in the drop-down menu. I suggest "ipynb" and "pdf".

[choldgraf]

sure thing - I agree, but I can't take a picture of the ipynb download because it doesn't exist yet :-)