docs: Document using models on the client only

[abdulawalarif]

Summary

Working with models covered serverOnly and field scope, but not that you can define models you only use in Flutter. There is no clientOnly flag. Place the YAML in the server package as usual; generating the class on the server as well is harmless.

Fixes serverpod/serverpod#3670

Changes

• Add a "Using models on the client only" section after the visibility examples.
• Show placing the .spy.yaml file in the server package, running code generation, and importing the class from the client package.
• Note that omitting table avoids creating a database table, and point to immutable: true and shared packages.

Reference

• Mention creating models that are used client-side only serverpod#3670

[abdulawalarif]

Thanks for the review.
Applied all four suggestions.

[developerjamiu]

Thanks for the contribution