Update GitHub Pages section

[apreshill]

trafficstars

This PR updates the GitHub Pages section (and splits off the existing section on Travis). Cl topics like Travis/GH actions remain to be updated.

[apreshill]

I agree- I think that would be much more user-friendly. This action exists: https://github.com/r-lib/actions/blob/master/examples/bookdown.yaml but focuses on Netlify.

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