Try https://voiden.md/

As per your needs it's: - Markdown (everything in markdown, including testing and specing APIs) - Let's you write your docs in markdown around the API you're designing/testing - Git-based (in-app terminal) - Code samples supported throug markdown

Plus offline, no login, no lock in, no telemetry.

It's early days so a tad rough around the edges still, but feels like the most natural way of handling APIs I ever used till date.