I've always been a fan of Dash the documentation browser for macOS. For many years, it's great UX that places documentation just a keyboard shortcut away has been a valuable tool in my workflow.
At the time I created rsdocs-dashing (August 2017), there weren't any great solutions to create docsets for Rust crates around.
Most other solutions just dumped the plain HTML output of rustdoc into a docset which results in a subpar experience.
What rsdocs-dashing does is quite simple. It takes output from cargo doc, edits the CSS to remove redundant elements (like the navigation sidebar) and puts it in a directory
together with a prepared dashing.json file, that defines what CSS selectors correspond to certain symbols (e.g. a trait or a function).
This allows you to then generate a docset via dashing.
In total, this allows for a workflow (that you can also put into a simple bash script) to turn crate documentation into a docset.
Nowadays, the project is mostly obsolete, as Dash has added a first-class integration for crate documentation on docs.rs. For projects that don't host their documentation there, it might still be a valuable starting point (though probably requires tweaking of CSS selectors).