The official Kali documentation is split into small sections and covers most of the topics you’re likely to bbe interested in, ranging from installing Kali to more advanced topics such as installing Docker in the distro. Kali offers an official training course that features a certificate on completion. There is a separate page with summaries of the included security tools with links to their respective pages.
We didn’t think that anything would beat the Kali documentation, but the Parrot Security website takes a similar approach in terms of splitting the docs up into small articles, but there seem to be slightly more articles and they tend to be a little longer.
On the Pentoo website, clicking on the Docs link leads to a page with a small amount of information about boot options and a nine-year-old YouTube video about the project. There is also a brief FAQ. The documentation is underwhelming and it’s obvious that the developers expect the user to rely on Gentoo documentation and forums.