Document the intended way to do composition

**Your Godot version:**
4.7.1

**Issue description:**
Document the intended way to do composition

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/getting_started/introduction/godot_design_philosophy.html
https://docs.godotengine.org/en/stable/tutorials/best_practices/scene_organization.html

Read the Godot design philosophy and scene organization pages linked in the issue, then identify where composition is discussed and what guidance is missing. Define the intended composition approach in documentation, with examples or cross-links only where the existing pages support them; done means the relevant guidance is clear and discoverable.

Written by the indexing model from the issue text.