Add support for deploys based on PRs to make it easier to test them

[george-gca]

Have you checked that your feature request isn't already filed?

• I read through FAQ and searched through the past issues, none of which addressed my feature request.
• Yes, I have checked that this feature request isn't already filed.

Description & Motivation

To have a deployed version of the site ready to be tested based on a PR would make it much simpler for us (maintainers) to validate them.

Pitch

No response

Alternatives

I was able to make it work for Netlify only.

Additional context

Suggested in #1981, made it work for a forked repo and shown in #2415.

Steps needed:

1. Create an account in Netlify
2. Add a Deploy with Git from this template (note: you should give access to Netlify to your repositories, but this can be limited by repo)
3. In the new deployed site, go to Site configuration -> Build & deploy -> Continuous deployment -> Build settings. Set the Build command as sed -i "s/^\(baseurl: \).*$/baseurl:/" _config.yml && bundle exec jekyll build and the Publish directory as _site. This can be seen in the screenshot below:

4. Enable Deploy Previews

[alshedivat]

@george-gca, that sounds like a great idea. let me set this up. also, appreciate all the features you've been shipping! 🚀

[george-gca]

The best way to test this is to create a PR with a really obvious change and see if after a while the Netlify bot adds the link to the built page in the PR.

[george-gca]

How is it going? Need any help?