Create documentation for "Architecture" section

[tigleym]

This issue looks at documenting the current state of space_shooter_rs's ECS architecture. This aims to be a helpful resource for both maintainers and new contributors when navigating the project's code. Ideally, this part should have a good balance between text content and visual aids.

Relevant files

• book/src/SUMMARY.md (update the relevant links)

• Create Markdown files for documentation