Add GitHub action to purge cache on update

[andrewda]

Currently, when a change is made to the website, it can take up to a few hours for the change to appear live for everyone on sert2521.org because of Cloudflare caching. Luckily, Cloudflare provides us with an API we can use to purge the cache for all or specific files. Ideally, the workflow would be as follows:

1. A change is committed to this repo and automatically published on our website
2. The GitHub action calculates the files that were changed in the last commit
3. The action uses the Cloudflare API to purge those files

This will make a much smoother developer and user experience. If you're interested in tackling this, let me know and I can help get you squared away with GitHub Actions and the Cloudflare API credentials!