[DOCS] Plugin system documentation and discoverability

[deucebucket]

Problem

The plugin system (custom HTTP layers + Python drop-in plugins) is fully implemented but poorly documented and discoverable:

1. Python plugins are invisible - no UI mentions them, no docs, example plugin buried in test-env/
2. No standalone documentation - users rely entirely on inline wizard hints
3. Secrets management unclear - UI asks for "secret key names" but doesn't explain secrets.json
4. No real-world examples - no sample configs for common book APIs
5. No distinction in UI between HTTP custom layers vs Python plugins

Scope

1. In-app help improvements (templates)

• Add a "Plugin Types" section to the Plugins settings tab explaining both HTTP layers and Python plugins
• Add a "Getting Started" or info card that links to docs
• Improve secrets management guidance in the wizard (explain where to put keys)

2. Documentation page

• Add a /docs/plugins route (or in-app docs modal) covering:
  • Quick start for HTTP custom layers
  • Python plugin development guide
  • Plugin lifecycle (setup, process, teardown)
  • Secrets management
  • Troubleshooting

3. Ship example plugin properly

• Move example plugin from test-env/ to a documented location
• Reference it from the docs/UI

4. Real-world examples

• Google Books API config walkthrough
• Open Library API config walkthrough

Out of scope

• Plugin marketplace (feat: Plugin marketplace / community registry (#66) #190) - separate feature
• New plugin capabilities - docs only