Reorganize documentation structure (CONTRIBUTING.md vs COLLABORATOR_GUIDE.md) #7836

As discussed on Slack, the documentation structure should be clear and intuitive, and not in large, hard-to-parse (for newcomers) files.

Our main issues are that:

1. COLLABORATOR_GUIDE.md contains general contribution information that should be accessible to all contributors, not just collaborators
2. CONTRIBUTING.md contains actual collaborator guidelines, which feels backwards
3. COLLABORATOR_GUIDE.md is too large and covers too many topics - it should be broken down into focused files.

With that in mind, following the pattern used by Node.js core (https://github.com/nodejs/node/tree/main/doc/contributing), we should break down our contributing guide into:

docs/
├── README.md (navigation guide)
├── getting-started.md
├── code-style.md
├── writing-tests.md
├── adding-pages.md
└── collaborator-guide.md
...

This will make the documentation more accessible and easier to maintain for both new contributors and collaborators.