Generate HTML preview links on PRs

[micahhausler]

It wouldn't be a ton of work to do a git diff --name-only $GITHUB_HEAD_REF $GITHUB_BASE_REF | grep ThreatModel.html and generate a list of https://htmlpreview.github.io/ links when new examples are added, and it'd be helpful for reviewers to just click the links instead of scrounging around to put them together

• https://docs.github.com/en/actions/reference/environment-variables

example link from #9: https://htmlpreview.github.io/?https://github.com/hyakuhei/fluentm/blob/d6595139c6f6e164b30634c1165668386ff366a7/examples/tinkerbell/ThreatModel.html