Add ability to preview computation from BinderBot in a GitHub PR without registering for a site hosting service

[choldgraf]

People often run their BinderBot execution in a Pull Request context. The GitHub action executes notebooks against a remote resource, and returns back the results of those computations. Currently, there's no way a person can easily preview these results within the PR.

We should make it possible for BinderBot to return a link to a previewable space for the notebook, so that users can quickly see the results in the PR.

It's already possible to do this with Netlify, ReadTheDocs, etc using the clinder CLI directly (http://2i2c.org/clinder/preview-deploys/). However, this requires an additional sign-up and configuration layer that might be too complex for communities that just want a quick way to demo the results of computation.

We should make it possible to preview the results of execution on a PR without requiring authors to sign up for a whole other documentation service.

Implementation ideas

This might not be the right repository for this! Ideally we can leverage another service for previews, so this could be either:

• A new open source tool or service, e.g., something like jupyterbook.pub, or something attached to a jupyterhub
• Pythia have a GitHub actions based workflow here that seems to work with clinder: Try switching to binder ProjectPythia/pythia-foundations#655