Outdated SDK documentation references deprecated imports and missing `StableCoinClient`

### **Description**

The SDK documentation for initialization is outdated and references packages and exports that no longer exist in the current version of the SDK.

The official documentation page also links to content that appears to be missing - see issue: https://github.com/hashgraph/hedera-docs/issues/509

---

### **Steps to Reproduce**

1. Navigate to:
[https://github.com/hashgraph/stablecoin-studio/blob/main/documentation/sdk/usage.md#initialization](https://github.com/hashgraph/stablecoin-studio/blob/main/documentation/sdk/usage.md#initialization)
2. Review the **Initialization** section
3. Observe the suggested imports:

```ts
import { Client } from "@hashgraph/sdk";
import { StableCoinClient } from "@hashgraph/stablecoin-npm-sdk";
```

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### **Observed Behavior**

* The SDK now uses `@hiero-ledger/sdk` instead of `@hashgraph/sdk`
* `StableCoinClient` is not exported in `@hashgraph/stablecoin-npm-sdk`
* The class may be deprecated or removed, with no clear replacement documented
* Documentation links to non-existent or outdated pages

---

### **Expected Behavior**

* Documentation should reflect the current SDK (`@hiero-ledger/sdk`)
* Deprecated or removed classes (e.g., `StableCoinClient`) should be:
* Removed from examples, or
* Replaced with the correct modern alternative
* All documentation links should resolve to valid, up-to-date pages

---

### **Environment**

* OS: Linux
* Browser: Firefox
* SDK version: 4.20.0