Improves mediator and middelware documentation

[PascalSenn]

No description provided.

[Copilot]

Pull request overview

Updates Mocha Mediator pipeline/middleware documentation and aligns the MediatorShowcase example code (and its source-generated registration) with the simplified middleware style and updated generator output.

Changes:

• Simplifies the middleware logging example to use async/await and removes the documentation section on the synchronous fast-path optimization.
• Updates MediatorShowcase pipeline behaviors to the same async/await pattern and removes now-unused System.Runtime.CompilerServices usings.
• Regenerates the MediatorShowcase DI extension to register handlers via AddHandlerConfiguration(...) with PipelineBuilder.Build*Pipeline(...) delegates.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

File	Description
website/src/docs/mocha/v16/mediator/pipeline-and-middleware.md	Simplifies middleware example and removes the sync fast-path optimization section.
src/Mocha/src/Examples/MediatorShowcase/PipelineBehaviors.cs	Converts showcase middleware to async/await and removes fast-path/local helper patterns.
src/Mocha/src/Examples/MediatorShowcase/MediatorShowcase.cs	Removes an unused using directive.
src/Mocha/src/Examples/MediatorShowcase/Generated/Mocha.Analyzers/Mocha.Analyzers.MediatorGenerator/MediatorShowcaseMediatorBuilderExtensions._5hYhW_IBO7W3L_MSoxCPw.g.cs	Updates generated registration to add handler configurations and pipeline delegates per handler.

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[PascalSenn]

LGTM