Same at Google, the most important thing is that the main documentation source internally is just markdown files checked in alongside code that is automatically

Same at Google, the most important thing is that the main documentation source internally is just markdown files checked in alongside code that is automatically served on the documentation site. Freshness is encouraged by owners periodically getting bugs to review pages and update a tag indicating the page was fresh as of X date. There's support for writing out diagrams in graphviz dot but people usually just draw them in google docs or some other external program and check a screenshot of that in to be embedded in the page. I'm sure some systems had more automation building documentation out of code directly but usually it's just the simple markdown written manually. There's also google docs which is nicer for quickly iterating and getting reviews but it's much less discoverable and falls out of sync with the system much more easily.