Build & Test Docs Site with CI & HTML Proofer

[FelicianoTech]

Hey,

I'm a big advocate for static docs sites and Hugo in particular (I'm a contributor). I see that this docs site is built with Hugo but not on CI.

I propose that this site gets built on CircleCI, this way PRs can be checked automatically before merging. There's a tool that I've used with Hugo and Jekyll-based websites called HTML Proofer that can test the generated static site for errors. My favorite usecase for it is to check for broken links though it can also check your HTML structure as well to make sure that well formed HTML pages are being generated.

A few examples of how this works:

Feliciano.Tech/config - my personal site, built with Hugo and tested with HTML Proofer
CircleCI Docs/config - CircleCI's documentation website, built with Jekyll and tested with HTML Proofer
Patheon Docs/config - Docs site for Pantheon Hosting, though they're using Gatsby instead of Hugo

I can get a PR going if this sounds good. Just wanted to ask/discuss first.