Static docs, Netlify, and PR branch deploy previews

[thompsongl]

Summary

Based on work done in #2577, I've got a PoC working that would allow us to:

1. Build the EUI documentation as a static site
2. Move our documentation to Netlify...
3. which allows us to offload the docs build to the Netlify server...
4. which allows us to automatically create doc deployments for all pull requests
5. Remove the docs/ directory from source control
6. Remove the build-docs script from the manual deployment process
7. Enable Elastic Site Search
8. Maintain versioned documentation sites (via subdomains; more research needed)

Assets

Working branch: master...thompsongl:static-docs
Deployed site: https://thompsongl-eui.netlify.com/
Netlify config:

Discussion

• We would be moving away from GitHub pages
  • Need a new, custom domain
  • Need to set up domain forwarding
• Netlify would be heavily involved in EUI operations
  • 💰?
  • Individual accounts?
  • https://www.netlify.com/legal/open-source-policy/?

[thompsongl]

It was a long break; just thought of one more thing: We'd need to tweak the release process a bit and/or use (potentially) GitHub actions to specify which tags/branches get deployed (aside from PR previews). By default, the master branch would always be the production deployment, which isn't how EUI releases work today. TBD here //cc @chandlerprall

[thompsongl]

I'll likely close this issue in favor of another more appropriate one, but it's doubtful that Netlify and the approach outlined above will be our eventual direction. We're beginning work with Elastic infra to set up a more custom/configurable solution backed by Elastic.

[thompsongl]

Closing this in favor of tracking in #2799