sig-node: CONTRIBUTING.md updates

/sig node
/kind documentation

The sig-node contributor documentation currently assumes a lot of existing knowledge about how the kubernetes project works. To help foster new contributions from people who don't necessarily have existing upstream contributor knowledge, or people within their organization who can train them we should update this document.

I think we probably need to focus on the following as a first step:

- How do you get the code and build/test a kubelet change
- e.g you likely need access to a linux machine/vm with containerd or cri-o installed, might not be obvious to a new contrib.
- How do you make a good first contribution
- How do you find issues?
- How do you request feedback?

Something to think about in a future community meeting would be talking about how we can help foster these contributions. Reviewer/Approver capacity is low, but we can't fix that without new contributors long term.

Update the sig-node CONTRIBUTING.md documentation, starting with the requested guidance for getting the code and building/testing a kubelet change. Cover finding issues and requesting feedback, including the Linux machine or VM and containerd or CRI-O context mentioned in the issue. Done means new contributors can follow these steps without existing upstream Kubernetes knowledge.

Written by the indexing model from the issue text.