Add plugins API endpoint documentation

[dannywillems]

Summary

• Document GET /api/plugins endpoint (list all plugins with event counts)
• Document GET /api/plugins/:name endpoint (get plugin details)
• Include response schemas, curl examples, and error responses

Test plan

• CI build passes
• Plugins page renders correctly in the API sidebar

[dannywillems]

Superseded by #31 which includes the plugins docs with tabs and Python examples.