Reproduce DR learner notebook using chirho.robust

[agrawalraj]

This PR closes #373

[SamWitty]

This notebook could certainly use some more text, but I think we can add that in a subsequent PR.

See my comment about the outline for an easy to address issue that should be fixed before the PR lands.

[SamWitty]

I believe the outline is taken from the backdoor adjustment tutorial, and doesn't align with this notebook.

[eb8680]

I think the outlines are wrong or at least have broken links in a number of other tutorials, which suggests the manually written outlines may be too brittle to bother maintaining when Sphinx, JupyterLab, VSCode and most other editors already generate and render outlines automatically from the section headers. I just removed it from this one entirely.

[SamWitty]

Sounds good. We can follow up in a separate PR to make all the outlines consistent with whatever we decide.