Add a section about handling Markdown notebooks

[stargaser]

• Add a section on handling Markdown notebooks
• Tell users to right-click then "Open With" then select "Notebook" or "Jupytext Notebook"
• Tell users how to set Document Manager settings to make double-clicking on a notebook file open it correctly
• Add an image of the Document Manager settings tab

Fixes #138
Fixes #143

[bsipocz]

Thank you @stargaser.

I don't know whether we think anyone uses jupyter notebook instead of a jupyerlab, so it's probably OK not to mention it in the docs (and anyway, that would work out of the box once jupytext is installed.

[bsipocz]

None of this should be responsible for the CI failures, so I go ahead and merge it. Thanks!