Document the shared feature composition architecture for maintainers and consumers

[nradakovic]

What

Document the new feature composition architecture introduced by this Epic, including how shared, platform-specific, and consumer-extensible features are separated and assembled.

The goal is to make the refactored model understandable and maintainable without requiring source-level archaeology.

Acceptance Criteria

• Maintainer-facing documentation explains the composition architecture and ownership boundaries.
• Consumer-facing documentation explains which extension points remain available for injected features.
• The docs clearly separate:
  • shared/common required features
  • Linux-specific required features
  • QNX-specific required features
  • consumer-extensible features
• The documentation matches the implemented refactored structure.

How

• Add or update architecture documentation for the toolchain feature model.
• Describe how platform templates assemble shared and platform-specific feature bundles.
• Document the intended role of extra_known_features and extra_enabled_features after the refactor.
• Link the new documentation from the relevant extension or validation docs where appropriate.