Document screenReaderSpeed accessibility updates introduced by PR #25

[Copilot]

This updates documentation coverage for PR #25, which introduced screenReaderSpeed-related source changes in UserSettings. The docs now reflect the new accessibility setting surface and its notification propagation path.

• Product documentation (PRODUCT.md)

  • Added screen reader speed to accessibility capabilities so user-facing feature coverage matches current plugin scope.

• Architecture documentation (ARCHITECTURE.md)

  • Updated notification-system description to include accessibility change forwarding for screen reader speed updates.
  • Added screenReaderSpeed key under accessibility settings in technical implementation details.

• Scope

  • Documentation-only change; no runtime behavior or API implementation was modified in this PR.

### Accessibility Features
- Adjustable voice guidance speech rate (0.1x to 10x)
- Screen reader speed preference for accessibility narration
- Voice guidance hints and prompts