Docs: update contributing guide for a general audience

[francisfuzz]

👋 Primer team!

This repo's Contributing Guide contains two internal references that are not open for a general audience.

First, @product-design is not a GitHub user. I'm assuming that this was meant to be an alias for @github/product-design or @primer/product-design, but either can only be mentioned if the caller is a member of either of those organizations:

https://github.com/primer/css/blob/master/docs/content/getting-started/contributing.md#step-1-open-an-issue

Second, there's a reference to #design-systems Slack channel for the GitHub workspace. This is not accessible to the general public either. 😅

I'd like to cut a PR for this to update these accordingly to reduce confusion. 😎