Mhh.. I believe https://orgmode.org/worg/ might be the first Git-based collaborative documentation ever: it started ~14 years ago, when GitHub wasn't here (remember?)
https://git.sr.ht/~bzg/worg/commit/d48bda02
Can you find previous example?
@bzg Ikiwiki was released in April of 2006, so there might be some git based wikis around that are a bit older. My own ikiwiki instance has been in operation since 2008, but it isn't really collaborative.
