Add to site usage on how to use Checkstyle as a Dependency

In response to https://stackoverflow.com/questions/63178064/why-is-checkstyle-not-in-maven-central ,

I can only find instances of downloading checkstyle , https://checkstyle.org/index.html#Download and https://checkstyle.org/cmdline.html#Download_and_Run , and nothing that states checkstyle is in maven or how to add it as a dependency. I understand maven-checkstyle-plugin is what actually runs checkstyle in maven, but we should atleast mention it is available in maven and how to add it as a dependency.

Review the existing Download and Command Line documentation linked in the issue, then locate the appropriate user-facing page for installation or usage. Done means the site states that Checkstyle is available through Maven and explains how to add it as a dependency, while the existing Maven plugin distinction remains clear.

Written by the indexing model from the issue text.