Make preview links on PRs more prominant?

[kescobo]

From @cmaimone on slack:

It can be a challenge for people who don't do PRs frequently to find the preview pages... If there were a way to somehow automate adding links to the previews for changed pages into a PR, that would be incredibly helpful

[kescobo]

Could possibly modify the actions from here: https://www.gostaticapp.com/jekyll-github-action

[peterfpeterson]

This would be a good bit of documentation to link into the PR template (in .github/PULL_REQUEST_TEMPLATE.md) so it pops up automatically