[Docs] Add Entity Component Manager (ECM) documentation example

[strangerwhoisharborofdoom]

The ECM (Entity Component Manager) is a core part of Gazebo Sim but lacks a beginner-friendly example in the documentation.

Suggestion

Add a minimal example showing how to:

1. Query entities using EntityQuery and ComponentTypeFilter
2. Retrieve component data using Component<T>
3. Create/modify entities at runtime

This would help new users understand the ECS architecture without digging into source code. I'd be happy to help contribute such an example if there's interest.

References

• Related discussion: https://github.com/gazebosim/gz-sim/discussions/XXXX