Separate contributing guide into practical advice and more philosophical bits

Currently, our `CONTRIBUTING.md` file contains mostly advice about what approach to take to writing code, how to format commit messages, etc. None of it is zerocopy-specific. We should add a section with pragmatic advice like "how do I update UI test .stderr files" or "how do I test on different toolchain versions". We have a good amount of bespoke infrastructure that has tripped people up in the past.