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

[developerjamiu]

Suggestion: drop immutable: true from this example. The page only introduces immutability further down, and the sentence below the example already points readers there when they need value equality.

Suggested change

immutable: true

[developerjamiu]

Suggestion: mirror how the intro of this page describes code generation, so both spots teach the same loop.

Suggested change

	Place the `.spy.yaml` file anywhere in the server's `lib` directory, run code generation, and import the class from the client package.
	Place the `.spy.yaml` file anywhere in the server's `lib` directory. With `serverpod start` running, saving the file regenerates the code; otherwise run `serverpod generate`. Then import the class from the client package.

[developerjamiu]

Suggestion: the other import examples in the docs use your_client, so keeping that name here helps readers connect them.

Suggested change

	import 'package:my_project_client/my_project_client.dart';
	import 'package:your_client/your_client.dart';

[developerjamiu]

Suggestion: this reads as if omitting table is what makes the server copy harmless; the two facts sit side by side rather than cause and effect.

Suggested change

	The class is also generated on the server. That is harmless: omit the `table` key so no database table is created, and you do not need to use the class in any endpoint. Set `immutable: true` if you want value equality for Flutter state management. See [Immutable classes](#immutable-classes).
	The class is also generated on the server, which is harmless: without a `table` key no database table is created, and nothing requires you to use the class in an endpoint. Set `immutable: true` if you want value equality for Flutter state management. See [Immutable classes](#immutable-classes).