Interact with Cloudflare API to purge cache after deployment

[MattIPv4]

Add a new script in the build directory that uses a Cloudflare API key & domain in the config file, and triggers a complete purge of the Cloudflare cache via the API.

In the build workflow, this can then be called after the build completes. This should hook into the result of the github-pages-deploy-action step (or the artifact check in #1) to only purge when a truly new build is deployed.