Update the documentation for Checkstyle.
Review Request #11777 — Created Aug. 10, 2021 and submitted
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 errors, spelling errors, and bad links.