Add Roslyn source generators conceptual overview and incremental generator tutorial

[Copilot]

Adds dedicated source generator documentation to the Roslyn SDK section—previously just a one-line external link to the cookbook. Aligned with companion sample code in dotnet/samples#7091.

New docs

• source-generators-overview.md — What source generators do, incremental pipeline model, input providers (ForAttributeWithMetadataName, AdditionalTextsProvider, AnalyzerConfigOptionsProvider), project setup, debugging, best practices
• tutorials/incremental-source-generator-tutorial.md — Step-by-step walkthrough building the GenerateMembers generator: marker attribute injection via RegisterPostInitializationOutput, attribute-driven filtering with ForAttributeWithMetadataName, Describe() + PropertyNames code emission

Snippet projects (compile-tested, output-verified)

The tutorial snippets mirror the GenerateMembers sample from dotnet/samples#7091:

[GenerateMembers]
public partial class Person
{
    public string FirstName { get; set; } = string.Empty;
    public string LastName { get; set; } = string.Empty;
    public int Age { get; set; }
}
// → Person.Describe(), Person.PropertyNames generated at compile time

Generator targets netstandard2.0 with Microsoft.CodeAnalysis.CSharp 4.10.0; demo targets net8.0.

Other changes

• index.md — Replaced external cookbook link with references to new docs + samples repo
• toc.yml — Added overview and tutorial entries under Roslyn SDK section
• Overview links to both samples: GenerateMembers (attribute-driven) and CsvGenerator (AdditionalTextsProvider)

Original prompt

I'd like to improve the documentation for Roslyn source generators. In particular, I'd like a conceptual document and a separate tutorial/user guide. Ideally we'd walk through the creation of a simple incremental source generator that the user can follow along with.

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