Moving contributor guide to expected location

[terriko]

Our contributor guide is currently found in https://github.com/intel/cve-bin-tool/blob/main/doc/CONTRIBUTORS.md

It looks like goodfirstissue.dev, Github, and others are starting to expect documentation for contributors to appear in /CONTRIBUTING.md

I can't really think of any reason to maintain a slightly different filename, so I propose renaming this guide to CONTRIBUTING.md and moving it to the root directory, then leaving a symlink in doc/ so that it will be picked up correctly by sphinx and build for Read The Docs. (similar to what we do for README.md right now) You'll need to double-check that the docs build correctly after that's done and that there's no broken links to the old doc.

This is probably a good first issue for someone who wants to learn to build the docs.

[ashok-arora]

@terriko Can I work on this?

[terriko]

@ashok-arora sure, have fun!