docs: Add proxy behavior example to Deep State tutorial

[atmosuwiryo]

Adds a concrete code example to clarify that mutations to $state() proxies don't affect the original object:

let original = [1, 2, 3, 4];
let numbers = $state(original);

numbers.push(5);  // numbers is now [1,2,3,4,5]
console.log(original); // Still [1,2,3,4] - unchanged!

Makes the existing NOTE section clearer for learners.

[vercel]

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[sacrosanctic]

Do we need an example for every statement on the docsite?

[atmosuwiryo]

You're right that we don't need examples for everything.

My concern was that the current code in the REPL let numbers = $state([1, 2, 3, 4]) doesn't actually demonstrate the proxy behavior mentioned in the NOTE - there's no 'original object' to show the separation from.

The NOTE felt disconnected from the REPL code.