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.

[cderv]

A few comments below because I don't understand some stuff.

Also, I wonder if it would be better to have a Netlify chapter, then two subchapter , one one Drop and one on deployement

6. Publishing
    |>> RStudio Connect
    |>> Netlify
        | >> Netlify Drop
        | >> Netlify Deployment 
    |>> Github
        |>> Github Pages
        |>> Using Github Actions

I understand you do that to offer option from simpler to more advanced, right ?

Maybe the publishing section needs further restructuring later on to organize by type of method or other instead of services. 🤔

[cderv]

This approach suppose that the built book is commited to Github in the main branch right ?

Source repo needs to have

• Source of the book to be able to rebuild the book
• Source of the website (build book) so that Netlify can retrieve and deploy (bc Netlify does not know how to build the book unlike with Blogdown and Hugo)

I am not sure this is clear to every user. This methods and Github Pages / Gitlab Pages without CI are the same.

Maybe we should mention such things ?

[cderv]

Why this is needed ?

Netlify lets you pick the folder in which the files for the static website are stored in git right ?

So any output_dir value would work, wouldn't it ?

[cderv]

Why do we need this for Netlify deployment exactly ?

These commands are to remove a folder from being tracked by Git without removing the folder. Usually when you want to untrack something and add it to gitignore.

Suggested change

	* Clean up committed files. If you placed a directory like `docs` in your GitHub repo, you need to do an extra step in the terminal to remove it with:<br>
	```bash
	git rm -r --cache docs/
	git commit -m 'Remove the docs folder'
	git push origin master
	```
	* Clean up committed files. If you placed a directory like `docs` in your GitHub repo, you need to do an extra step in the terminal to remove it with:<br>
	```bash
	# Tells git to stop tracking the docx/ folder but does not delete this folder
	git rm -r --cache docs/
	git commit -m 'Remove the docs folder'
	git push origin master

Also maybe you should keep using `main` instead of `master` for examples also ? 

[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)

[yihui]

Like #1218 (review), I feel both the documentation and usage will become easier after we provide an action or a function publish_netlify(). Users would only need to provide a site id and personal access token, and we can take care of the rest of technical details.

[apreshill]

I would LOVE to document that ;)

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