Ask HN: software for project documentation?

I've taken over a legacy project that has no documentation, and no documentation infrastructure.

I need to start creating documentation and set up best practices. This isn't about per-class, per-method documentation, but covering the architecture of the web app, its style guide, where Redis is used etc.

Setting up Confluence would have too much overhead (as they don't see the need, they don't want to use my time for that). They're not using GitHub, so GitHub wikis are out (though not my preferred choice either).

Last time I looked for solutions was c. 2008 when dokuwiki was popular; what's appeared since? I am aware I could use virtually any CMS or Wiki, but I'd like to know what's worked for other teams.

Thanks!