Add a contributor guide

[bjohansebas]

Unlike the contribution file, we could have a contributor guide that better explains the process for editing certain things on the page, similar to the guide Node.js has on its website. We already have part of that work done, we just need to locate and explain it better. What do you think?

ref: https://github.com/nodejs/nodejs.org/blob/main/COLLABORATOR_GUIDE.md