docs: add signals quickstart guide

[jlugao]

Summary

• Added comprehensive signals quickstart guide (docs/signals.md)
• Updated mkdocs.yml to include new signals documentation in the navigation

Changes

• Created docs/signals.md with examples for:
  • Available signals table
  • Basic logging hook example
  • Performance monitoring example
  • Session monitoring example
  • Multiple callbacks and disconnecting signals
  • Common use cases

Closes #49