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?)
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.
@bremner True that, thanks for mentioning it.
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