The styling looks familiar from Meson's website at https://mesonbuild.com/, and checking Inspect Element on Meson's site reveals hotdoc references in the HTML source. Though I still don't know if it's better than other docs/website frameworks.
It is the framework used by meson which was how I came across it. It's different, for short API documentation the yaml structure probably works better.
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[ 12.4 ms ] story [ 135 ms ] threadSimilar to Antora which you will probably recognise, though Antora also draws content from multiple Git repositories.
https://antora.org/ (not built with Antora)
Showcase of sites using it: https://gitlab.com/antora/antora.org/-/issues/20
(I'm just a user.)