docs: RFC - restructure plugin documentation

[zkdiff]

Summary

Proposing a restructure of the plugin documentation. Looking for feedback on direction before investing more time.

Changes

• Split plugin docs into a dedicated "Plugin Development" page
• Reorganized content into three sections: Events, Hooks, and Providers
• Added detailed examples for events (session, message, file, system)

Context

Started this while working on improving the anthropic auth plugin flow, noticed the plugin docs could use some reorganization. Before polishing the examples and verifying all the details, wanted to check if this direction is useful.

Questions for maintainers

1. Is this level of detail/structure helpful, or is it overkill?
2. Does the Events/Hooks/Providers organization make sense?
3. Any preferences on how event examples should be formatted?

Happy to iterate based on feedback or close if this isn't a priority.