Add a demo for use in github actions

[trevorcampbell]

Perhaps create an action for this that people can use, or a demo with a custom-coded yml to show people how to use website_diff in GH actions to automatically produce html diffs. Maybe just use this in the DSCI100 textbook repos and point to that?

[joelostblom]

A GH action template somewhere would be helpful for quickly trying it out in CI elsewhere!

[trevorcampbell]

Initial example is here:

https://github.com/UBC-DSCI/introduction-to-datascience-python/blob/main/.github/workflows/deploy_pr_preview.yml

results are here:

https://python.datasciencebook.ca/diff350/index.html

Still need to make a few minor fixes (#14, #15, #16) but otherwise it should be good to go :)