Replace functions / classes with links to source documentation

[adrn]

This is only relevant once #153 is merged

sphinx-gallery has a nice feature in which it auto-identifies function and class names from intersphinx-ed packages and replaces the rendered html versions of the names with links to the source documentation. For example: check out the last code block on this page.

We might be able to use the sphinx-gallery machinery to (hackily) do this to our own rendered notebooks as a postprocessing step during the sphinx build. This would likely require modifying nbsphinx to support specifying a custom nbconvert postprocessor class (that we'd have to write).