Enable Netlify integration (preview)

[arm4b]

Research enabling Netlify integration so someone creating or reviewing PR would see a preview of the pages live before merging. Will help a lot with development.
We won't deploy stackstorm.com via Netlify as GH Actions are responsible for deployment to GH Pages, just would use as a preview platform.

I recommend forking this repo and playing with the Netlify integration in the fork first. This would allow demonstrating the working state and recommending the configuration to st2 senior maintainers to set up account on the st2 side afterwards.

[arm4b]

Netlify Preview was enabled.
Example: #16 (comment)

Which shows preview for every PR both as a comment and build status check.