Add a date or commit ID somewhere to published HTML

[StevenMaude]

@iaindillingham spotted that the docs weren't being published.

We only had an email notification of failure. Render doesn't seem to display a publish check here on GitHub. (I checked on Render and not sure there's anything there to configure.)

Cloudflare Pages does display status here, for when we switch back, #754, but it might still be useful to have an easy validation in-page that content is current. And useful for readers too, as a warning that content may be old.