Add .NET async internals documentation with Mermaid diagrams

[Dafydd-E]

Comprehensive documentation covering:

• ExecutionContext and AsyncLocal internals
• SynchronizationContext and thread affinity
• Task scheduling and TaskScheduler hierarchy
• ThreadPool internals including HillClimbing algorithm
• Work-stealing queues and worker thread lifecycle

{PR title}

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Summary of the changes (Less than 80 chars)

Description

{Detail}

Fixes #{bug number} (in this specific format)

[Dafydd-E]

I'm testing out some AI tooling, apologies