Adopt PR previews?

[zklaus]

We could use the action https://github.com/rossjrw/pr-preview-action to add previews to all PRs. In practice, that means that the changed code from the PRs is rendered, published under a special path, and linked from the PR in a sticky comment. When the PR is closed (or merged) the preview is removed.

Here, I propose to adopt a third-party action; this is because the functionality is still missing from core github pages. However, it is being developed also there and maybe a replacement in the future.