Add GitHub workflow to show diff between HTML pages on push/PRs

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This changeset implements a simple workflow to report the diff between the PR base and head. You need to manually check the build status for details.

In the future, we could enhance this workflow to report back in a more visual way, such as replying with a comment on a PR.

I've toyed around with for a couple of hours but will leave this as-is for now with this link dump:

• https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-commands-for-github-actions (annotations for warning/error in build details)
• https://developer.github.com/v3/checks/runs/#annotations-object (annotations technically support "notice")
• Are notice annotations currently supported? actions/runner#707 (notice level annotations not supported via output, API should work)
• Multiline support for Toolkit Annotations actions/toolkit#193 (multiline output confirmed to be supported via %0A)
• https://github.com/marketplace/actions/comment-pull-request#disclaimer (add PR comment via github-script?)
• https://github.com/marketplace/actions/add-pr-comment#proxy-for-fork-based-prs (proxy needed for fork-based PRs?)
• https://time2hack.com/test-coverage-label-with-github-actions/ (PR label might be a reasonable alternative?)