Create a GitHub Action to automate deployment

[benfrancis]

Currently new versions of the documentation are deployed to GitHub Pages with the mkdocs gh-deploy command.

It might be convenient to automate this with a GitHub action so that commits on master trigger deployment automatically.

[benfrancis]

This can remove the need to use the gh-pages branch altogether, see https://docs.github.com/en/pages/getting-started-with-github-pages/configuring-a-publishing-source-for-your-github-pages-site#creating-a-custom-github-actions-workflow-to-publish-your-site