One thing there isn't enough of is task-driven documentation. Typically there's a getting started guide and a reference manual, with a huge gap between the two.
The getting started project is usually at the hello world level - ok as far it goes, which is not far.
The ref manual typically has a list of all moving parts but often lacks a list of concepts and terminology. So you look up $thing and have no idea how it fits into what you're attempting to do.
A series of mid-level task-based examples ("How do I...") for the most common use cases goes a long way to bridging that gap.
Also list any platform specific quirks. (Not just "To install this use homebrew $name for MacOS and apt-get $name for Ubuntu.")
And run a forum where people ask questions. Collect the most common questions into a literal FAQ and/or include them in examples/reference.
Never say "That's in the manual." Especially not when the manual is 1000 pages long and gives a two sentence description of every feature with no use cases.