Hotdoc's own bootstrapped documentation
You're looking at it, check https://github.com/hotdoc/hotdoc/tree/master/documentation for the sources.
GStreamer's documentation has been ported from docbook to hotdoc + markdown. Its API reference is currently being ported as well, using hotdoc's subproject nesting capabilities, see here for a snapshot.
The meson documentation is a nice showcase of hotdoc for a simple standalone project.
WIP GNOME developer portal
Some initial work has been done to port over the GNOME developer portal, with WIP ports of GLib and GStreamer nested as subprojects, see here for a snapshot.
Apertis is a Debian-based distribution for In-Vehicle-Infotainment. The documentation for its individual libraries is currently listed with the default HTTP index, it will hopefully be ported to use hotdoc's nested subprojects at some point.
Endless Modular Framework documentation
A tiny test project
The documentation generated for a project that tries to use as much of hotdoc and its extension's features while staying as tiny as possible can be found here, and its sources, which can be used as an example, live there
The results of the search are