Update docs coverage and hooks reference

[brunoborges]

Summary

Improve SDK documentation coverage alignment with current implementation and add discoverability links for newly documented sections.

Changes

• Update event reference docs to include SessionContextChangedEvent (session.context_changed).
• Fix hook API documentation drift in hooks.md:
  • Correct record accessor names (prompt(), source(), reason()).
  • Correct getToolResult() type to JsonNode.
  • Add startup to session start source values.
• Add docs for:
  • SessionConfig.clone() behavior and usage example.
  • SessionHooks.hasHooks() usage example.
• Add navigation links:
  • TOC link for Cloning SessionConfig in documentation.md.
  • Overview link for Checking Whether Hooks Are Registered in hooks.md.
• Update .gitignore to ignore blog-copilotsdk/.

Testing

• Validation scope: documentation and .gitignore changes only.
• Build/test execution: skipped (mvn clean verify) because no Java source/test files changed.
• Verified by inspection that links and section anchors are present in updated markdown files.