Add using statement guidance for extension methods in LINQ tutorial

[Copilot]

The "Working with LINQ" tutorial was missing explicit guidance about namespace requirements when creating the Extensions.cs file, causing compilation errors for users following the tutorial outside of Visual Studio.

Problem

When users follow the tutorial and create the Extensions.cs file with the LinqFaroShuffle namespace, extension methods like InterleaveSequenceWith are not accessible in Program.cs without a proper using statement. This works automatically in Visual Studio, but fails in other editors like Visual Studio Code.

Solution

Added two targeted clarifications to the tutorial:

1. Early guidance (after initial using statements): Added a tip explaining namespace organization options
2. Specific guidance (after Extensions.cs code): Added a note explaining when the using LinqFaroShuffle; statement is needed

Testing

Verified both approaches work correctly:

// Approach 1: Using statement (for top-level programs)
using System;
using System.Collections.Generic;
using System.Linq;
using LinqFaroShuffle;  // This line needed for extension methods

// Approach 2: Same namespace (traditional structure)
namespace LinqFaroShuffle
{
    public class Program { /* ... */ }
}

The changes are minimal and surgical - only adding clarifying notes without modifying any existing code or breaking changes.

Fixes #18562.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/tutorials/working-with-linq.md	docs/csharp/tutorials/working-with-linq

[BillWagner]

This is great. I'll mark it ready for final review.