There’s two things going on here.
First, the link is meant to be added to the PR body by a GitHub workflow:
It doesn’t care if a preview is even built, it just adds a link for docs PRs. For some reason it’s not always adding the link, we’ll have to investigate.
Second, Read the Docs builds the preview. This happened for both these PRs. If you scroll down to the bottom where it lists the checks, one of them is “docs/readthedocs.org:cpython-previews”, and you can click “Details” to see the preview.
For the second PR, it is: 3.13.0a1 Documentation