ability to preview HTML from pull requests online

[pdurbin]

@raprasad and I talked about how it would be nice to be able to preview the HTML from pull requests online while reviewing them. This would save reviewers the trouble of building the branch locally.

I asked about this in Write the Docs Slack ( https://www.writethedocs.org/slack/ ) today and was told that Netlify is a good option and the free tier might be enough for us.

Here's a pretty compelling gif from https://www.netlify.com/blog/2016/07/20/introducing-deploy-previews-in-netlify/

More details at https://www.netlify.com/products/deploy-previews/

The Toolsday podcast is always saying good things about Netlify, most recently at https://spec.fm/podcasts/toolsday/wKcuYe6l