Add desktop Feature Manifest reference and reorganize feature definition

[jaredlockhart]

The desktop FeatureManifest.yaml schema has no dedicated reference documentation. The only documentation is a brief 10-line example in feature-api.mdx. Meanwhile, technical-reference/feature-definition.md is a thin (37-line) generic page that doesn't clearly direct engineers to the right platform-specific docs.

New doc: Comprehensive Desktop Feature Manifest Reference covering:

• All feature-level properties (description, owner, hasExposure, isEarlyStartup, allowCoenrollment, applications, schema validation)
• All variable properties (type, fallbackPref, setPref, enum)
• setPref vs fallbackPref comparison table
• Variable types (boolean, string, int, json) with examples
• Real-world examples from the actual manifest

Reorganization:

• feature-definition.md rewritten as a crossroads page pointing to Desktop manifest reference, FML spec, and Cirrus guide
• feature-api.mdx registration section slimmed down with cross-link to full reference