I like manual pages, it's a native documentation format in a Unix environment. Using man(1) and apropos(1) is more convenient than online html pages in a web browser.

First of all OpenBSD has nice written manual pages for every component of the operating system.

Documentation for other projects formatted in mandoc:

Lua 5.1 C API manual pages
https://github.com/ligurio/lua-c-manual-pages

C++ 98/11/14 manual pages for Linux/MacOS
https://github.com/aitjcize/cppman

x86 and amd64 instruction reference manual pages
https://github.com/ttmo-O/x86-manpages

Formatted C++20 stdlib man pages (cppreference)
https://github.com/jeaye/stdman

How fast can a human possibly run 100 meters?

https://bigthink.com/starts-with-a-bang/fast-human-run-100-meters/