integrated vs. standalone documentation
There are multiple levels of "integrated vs. standalone vs. modules." Some boost docs are built within the superproject and the html goes into the top-level boost doc/ folder. More modular docs output their own set of html into libs/_my_lib_/docs.
how one does an "integrated documentation"
The general idea is that for separate modular docs (which is preferred), all you need to do is check out that particular library repository and build those. However "integrated documentation" is more complicated, it involves all the libraries, and many toolsets. Run https://github.com/boostorg/release-tools scripts in a container. Instructions: https://github.com/boostorg/release-tools/blob/develop/docs/README.md However, since this is automated and the results are published each day: https://www.boost.org/doc/libs/develop/ it's not necessary for a user to build in that way.