Move GitHub Pages build and deploy function into GitHub Actions

[gene1wood]

Move GitHub Pages build and deploy function into GitHub Actions. Probably also good to move the deploy from /docs/ to the gh-pages branch to avoid all the commits into the codebase which aren't code changes but just deployed artifacts.

[janbrasna]

Ultimately you can skip committing to gh-pages completely and have the action artifacts deployed to github-pages environment without any underlying git history et al., I'll look into some starter workflows for webpack (or just npm) what's usually used today now when the #215 update brings compatibility with newer node runners.

This means that any merged PR goes straight into production. (No need for maintainers to build locally and test — which also means PR preview deployments are much needed alongside this. Too bad GH still hasn't published theirs actions/deploy-pages#180 :/…)