jupyter: support relative links between documents

[williamstein]

Support your file layout is this:

A/a.ipynb
B/b.ipynb

Using this in b.ipynb should open a.ipynb but it does not:

[link to a](../A/a.ipynb)

With Jupyter this ends up opening B/A/a.ipynb.

Note that exactly this works fine between markdown documents, so there is something wrong about the parsing of relative links specifically with our Jupyter markdown renderer.

This is a problem with our markdown renderer in static mode. The slate WYSIWYG markdown renderer actually works properly regarding these links.

[williamstein]

WORKAROUNDS:

• Absolute links work fine, e.g., https://cocalc.com/.../A/a.ipynb
• Put a.ipynb and b.ipynb in the same folder.