Split preview CI

[ia3andy]

example: https://github.com/quarkusio/quarkusio.github.io/pull/1337/files

[holly-cummins]

Hm, I've had another thought. I did a quick google to see if they've invented a restricted-scope token that works on PRs from forks yet. People are asking for it, but they haven't done it yet.

However, it seems like it's possible to put arbitrary markdown in the action summary, and that does get appended to the PR, so it might meet our needs, without the clutter of multiple workflows: https://github.blog/2022-05-09-supercharging-github-actions-with-job-summaries/