Migrate Jenkins Clover Plugin documentation to GitHub

As per https://groups.google.com/g/jenkinsci-dev/c/lNmas8aBRrI/m/eL3u7A6qBwAJ the wiki.jenkins.io is in the read-only mode. Content served on https://plugins.jenkins.io/clover/ is served from this wiki. The recommended approach is to keep plugin documentation in the code repository. Details:

https://www.jenkins.io/blog/2019/10/21/plugin-docs-on-github/

