Use GitHub Actions to deploy GitHub Pages

[seshrs]

Context

Now that GitHub Pages supports deployments via GitHub Actions,, this commit:

• Updates the README to recommend deploying Primer Spec websites using GitHub Actions.
• Uses GitHub Actions to deploy this repository's website.

Using GitHub Actions gives us two benefits:

• We can add arbitrary steps to the GitHub Actions build step (https://github.com/seshrs/build-primer-spec-action). This means that we can theoretically add arbitrary Jekyll plugins, generate PDFs (#136) and more!
• [Minor] This repository can make develop the main Primer Spec branch for PRs.

Note: This PR is labeled as a "minor" change because these changes will not break course websites that use Classic GitHub Pages.

Next Steps

Ideally, the Primer Spec Preview workflows should update to use the custom build-action step also, but that will require adding some config options to the action. For now, we'll just need to remember to keep the actions steps in sync.

[github-actions[bot]]

The spec from this PR is available at https://preview.sesh.rs/previews/eecs485staff/primer-spec/212/.

^{(Available until Wed Oct 26 2022.)}

[awdeorio]

This is very cool!