Rebuild TaskFlow documentation

[dombrovsky]

What changed

• add a goal-oriented GitHub Pages documentation tree under docs/
• rewrite the repository and package-specific READMEs around TaskFlow's owned FIFO execution-lane positioning
• document lifecycle, cancellation, timeout, throttling, execution models, extension ordering, DI, compatibility, and troubleshooting
• add package-specific NuGet descriptions and discovery tags
• add a lightweight GitHub Pages workflow that builds the Jekyll site on pull requests and deploys it automatically from main

Why

The previous root README made broad performance, concurrency, and disposal claims and did not provide comprehensive guidance for TaskFlow's production usage patterns or edge-case semantics. Issue #19 also requires the long-form documentation to have stable Pages URLs and automatic publication.

Impact

This change updates documentation, package metadata, and Pages automation only. It does not change runtime APIs or behavior.

Validation

• Release build succeeded with zero warnings and errors
• 464 test executions passed across .NET 8 and .NET 10
• all 44 C# snippets extracted from 19 Markdown files were compiled during one-time release-remediation validation
• local links, anchors, front matter, canonical permalinks, and packaged NuGet metadata were checked during one-time validation
• the Pages workflow builds the complete Jekyll site on pull requests before deployment from main

Closes #19