We want to ensure that our documentation for contributors is of high quality and accessible so that new contributors can start working on our repositories.

Link to the contributor guide from c/common in Netavark, Aardvark and libhvee. Simple change, just add CONTRIBUTING.md, which points to c/common, and link in the readme. Also change c/storage and c/image to use the referenced c/common contributor guide. To reduce maintenance costs.