docs: add UserSettings documentation for display brightness automation and screen timeout APIs

[Copilot]

PR #62 adds new display-related settings in UserSettingsImplementation for manual/automatic brightness control and screen timeout, but the repository documentation did not describe them. This update closes that gap by extending the product and architecture docs to cover the new settings, defaults, and validation rules.

• Product documentation

  • Adds the new display settings to PRODUCT.md
  • Documents user-facing behavior for:
    • displayBrightness
    • autoBrightnessMode
    • screenTimeout
  • Captures defaults and constraints visible in code:
    • brightness default: 75
    • auto brightness default: false
    • screen timeout default: 300
    • timeout valid values: 0 or 30-3600

• Architecture documentation

  • Updates ARCHITECTURE.md to include the new settings in the managed settings inventory
  • Describes the interaction between manual brightness and auto-brightness mode
  • Notes the persistence/validation behavior implemented in UserSettingsImplementation

• API coverage

  • Documents the newly added implementation APIs and expected value ranges
  • Clarifies that enabling auto-brightness does not reject manual brightness writes, but manual brightness is ignored while auto mode is enabled

• Repository overview

  • Replaces the placeholder README.md content with a minimal overview that points readers to the detailed product and architecture docs for the new capabilities

Example of the newly documented API surface:

Core::hresult SetDisplayBrightness(const uint32_t brightness);
Core::hresult GetDisplayBrightness(uint32_t& brightness) const;

Core::hresult SetAutoBrightnessMode(const bool enabled);
Core::hresult GetAutoBrightnessMode(bool& enabled) const;

Core::hresult SetScreenTimeout(const uint32_t seconds);
Core::hresult GetScreenTimeout(uint32_t& seconds) const;