Add docs on authoring and using generator plugins for the C# emitter

[Copilot]

The plugins emitter option (added in #10249) lets emitter authors load custom generator plugins, but there was no documentation describing how to author or use them. This adds a guide to the existing packages/http-client-csharp/docs.

Changes

• New docs/plugins.md covering:
  • Plugin discovery: automatic (via node_modules dist) vs. explicit (via the plugins option).
  • Authoring a GeneratorPlugin subclass and the CodeModelGenerator extension points (AddVisitor, AddRewriter, AddMetadataReference, AddSharedSourceDirectory), plus a LibraryVisitor example.
  • A minimal plugin .csproj that references the generator via a NuGet PackageReference to the published Microsoft.TypeSpec.Generator.ClientModel package (which transitively brings in Microsoft.TypeSpec.Generator).
  • tspconfig.yaml usage and path-resolution behavior (paths absolute or relative to the resolved emitter-output-dir, auto-build of .csproj, DLL scanning).
• emitter.md — plugins option now links to the new guide and includes the inline path-resolution example.
• index.mdx — adds a "Generator plugins" section linking to the doc.
• emitter/src/options.ts — expanded the plugins option description to document path resolution (absolute or relative to the resolved emitter-output-dir) with an inline YAML example, applying the inline examples from docs(http-client-csharp): document plugins option path resolution with examples #10908.
• readme.md and website reference emitter.md — regenerated to reflect the updated option description.

Plugin authoring example:

using Microsoft.TypeSpec.Generator;

public class MyPlugin : GeneratorPlugin
{
    public override void Apply(CodeModelGenerator generator)
    {
        generator.AddVisitor(new MyLibraryVisitor());
    }
}

options:
  "@typespec/http-client-csharp":
    plugins:
      - "codegen/MyPlugin.dll" # file relative to emitter-output-dir
      - "codegen" # directory containing plugin assemblies
      - "/abs/path/to/MyPlugin.dll" # absolute path used as-is

Content was cross-checked against GeneratorPlugin.cs, GeneratorHandler.cs, CodeModelGenerator.cs, and the emitter options.ts/emitter.ts.

[jorgerangel-msft]

@copilot apply the inline examples from #10908

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/http-client-csharp@10907

commit: 9121f26

[github-actions]

No changes needing a change description found.

[Copilot]

@copilot apply the inline examples from #10908

Applied the inline plugins option examples from #10908 in 4302df0. Updated the option description in emitter/src/options.ts (path resolution + YAML example), propagated to readme.md, the website reference emitter.md, and docs/emitter.md, and reflected the same path-resolution behavior and example in docs/plugins.md. Also added a changeset for the option description change.

@copilot apply the inline examples from #10908

Correction: the change is in commit 201b792.

[azure-sdk-automation]

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