Add docs-dev script for running mdbooks locally

[Zachatoo]

To run this script requires installing mdbooks following the steps here https://rust-lang.github.io/mdBook/guide/installation.html

We may want to consider adding some documentation (in the README or elsewhere) to explain that there needs to be an installation process first.

I don't think we'd want to add an script to install mdbooks, since we may not know where the user wants to install the binary, but if y'all feel otherwise I can add an install script, just think it needs some discussion first before we add that.