Document the supported C++ toolchain feature catalog for consumers

[nradakovic]

What

Create consumer-facing documentation that explains which C++ toolchain features are supported, which are enabled by default, which are platform-specific, and how consumers can extend the feature model.

The goal is to make the explicit feature model understandable without requiring consumers to read toolchain implementation code.

Acceptance Criteria (DoD)

• A dedicated feature reference document exists for consumers.
• The documented feature list matches the implemented explicit feature set.
• The documentation clearly distinguishes:
  • built-in toolchain features
  • optional features
  • platform-specific features
  • consumer-injected features
• Consumers can understand how to use extra_known_features and extra_enabled_features.

How

• Create a feature catalog document that lists each supported feature and its behavior.
• For each feature, document:
  • name
  • default enabled state
  • platform scope
  • affected actions or behavior
  • whether consumers may inject or enable it externally
• Link the feature catalog from the existing extension and validation documentation.
• Keep the documentation aligned with the implementation and validation outputs from the related tasks.

Out of Scope

• Release communication planning.
• Downstream migration advice beyond the feature reference itself.
• Internal-only implementation notes that do not help consumers.