add netlify continuous deployment section

[apreshill]

This PR adds a section on Netlify continuous deployment, right after a section on Netlify Drop, to the publishing chapter of the book.

[apreshill]

Re: the organization. What you propose was how I did have it, but then the subsections end up heavily nested. And folks who use Netlify won't find the GitHub Actions section. What I would do:

6. Publishing
    |>> RStudio Connect
    |>> Netlify Drop (no CI)
    |>> Netlify Continuous Deployment (no CI)
    |>> Github Pages (no CI, to `docs/`) _[this and the above section could be fused- maybe?]_
    |>> Continuous integration (Github Actions to deploy to branch, maybe leave off Travis)
           |>> show both Netlify + GH Pages 

[cderv]

Yes I agree I think this makes sense. I would add rstudio connect on the CI part because this is what we use for our books and it is working quite well. (However, it requires maybe some simplification - needs to find time to document and update publish_book() function)

[apreshill]

I would LOVE to document that ;)

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