Update the documentation for Cargo Tool.
Review Request #11776 — Created Aug. 10, 2021 and submitted — Latest diff uploaded
This fleshes out the docs, listing the file types that are supported,
adding installation instructions for the tool, and covering all
configuration options (both worker-side and in Review Board). This
should help users get set up for the first time.
Built the docs. Checked for build and spelling errors and bad links.