feat(learn): split inclusions content into subcategory pages

- Replace single inclusions.yaml with dedicated subdirectory
- Add 8 focused pages: overview, solid inclusions, fluid inclusions,
  growth features, feathers, silk, natural vs synthetic, surface features
- Enable deeper navigation hierarchy for inclusions topic
- Support new subcategory field in content schema

Refs: gemmology-dev/gemmology.dev subcategory implementation

Author: Bissbert