Add dedicated documentation site for Good First Issue CLI

I noticed the current documentation lives entirely in the README, which can be limiting for new users trying to onboard quickly.
I built a dedicated Docusaurus-based documentation site that covers:

- Quickstart guide
- Command reference
- Automation and scripting
- Contributing guide
- Troubleshooting

You can find the site here: https://good-first-issue-docs.vercel.app/

Start by reviewing the current README and the linked Docusaurus site at https://good-first-issue-docs.vercel.app/. Compare the proposed quickstart, command reference, automation, contributing, and troubleshooting content with the repository’s existing documentation, then clarify how the site should be incorporated before defining completion.

Written by the indexing model from the issue text.