docs: add fork support documentation for PR review workflow

[jerop]

• Add comprehensive section on extending PR review workflow to support forks
• Document simple fork support approach using contributor's own Google auth
• Explain GitHub Actions security model for fork-based PRs
• Provide implementation approaches from simple to advanced
• Include security best practices and resources for pull_request_target
• Reference centralized authentication documentation
• Reorganize content with clear implementation approaches

Fixes #191

[gemini-cli]

🤖 Hi @jerop, I've received your request, and I'm working on it now! You can track my progress in the logs for more details.

[gemini-cli]

📋 Review Summary

This pull request adds excellent documentation on how to extend the PR review workflow to support forks. The new section is well-written, comprehensive, and provides clear implementation approaches.

🔍 General Feedback

• The documentation is clear and easy to understand.
• The inclusion of security best practices and resources is a great addition.
• The different implementation approaches are well-explained, allowing users to choose the best option for their needs.

[leehagoodjames]

Hi Jerop,

I think this currently reads as an authoritative stance on workflow triggers and GitHub credential access policies. I think it may be more concise, and more inutitive, to instead explain our opinionated approach (explaining our "why" for not running on branches from forks), and then point the user to the docs if they want to deviate from that.

This summary would contain less breadth - while still covering the exact reasoning behind our decision - which some users may share