For the package manager ecosystem it supports, it relies on the projects auto-generated docs, and the builds the docsets from those. I guess it does that step in a cached on-demand way. That way, it can provide docs to all the packages and package versions.

E.g. for Rust: Crate is published crates.io -> triggers automatic docs build on docs.rs -> Dash clients can now pull docsets through a proxy that builds the docsets built on the static HTML bundles built for docs.rs.