Finish deploy actions business

[brian-rose]

Copied from #25, needed to merge that before everything was done in order to trigger the right actions.

• Use a minimal jupyter-book environment for the link checking step
• Use same minimal environment to build book previews, deploy to netlify
• Build and deploy to gh-pages upon push to main branch
• Put useful status badges on README page
• Update build instructions

[brian-rose]

Haven't got the Netlify previews working yet.

[brian-rose]

I'll leave that for a future update.

Where things stand now:

• The book will self-publish to GitHub Pages whenever main branch is updated
• README shows useful status badges for the build and the link-checker
• Instructions have been updated to reflect this.

What's left to do:

• Preview the rendered book in Pull Requests, just like we do for Pythia Foundations