Clarify WeatherApiClient is part of Aspire starter template

[Copilot]

The documentation references WeatherApiClient throughout but never explains it's auto-generated by aspire new aspire-starter. Users might assume they need to create it manually.

Changes:

• Added note after template command explaining WeatherApiClient is template-generated
• Added inline comment in Quick Start code sample clarifying origin
• Updated authentication flow step 4 to mention the typed client is from template
• Clarified section 4.4 example extends the template-generated class
• Fixed spacing in return type: WeatherForecast[]? > → WeatherForecast[]?>

Example of added clarification:

> **Note:** The Aspire starter template automatically creates a `WeatherApiClient` class 
> in the `MyService.Web` project. This typed HttpClient is used throughout this guide to 
> demonstrate calling the protected API. You don't need to create this class yourself—
> it's part of the template.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[jmprieur]

It's a good PR, and I'll take the content aside.