docs: Enhance documentation with tutorials and examples for external developers

[kcenon]

What

Enhance thread_system documentation with tutorials, expanded examples, FAQ, and troubleshooting guide for external developers.

Why

thread_system has 19 example directories but lacks guided tutorials explaining when and how to choose between thread pool types, queue implementations, and scheduling strategies. External developers need decision guides and step-by-step walkthroughs.

Where

File	Action
docs/tutorial_threadpool.dox	Create — Thread pool selection and usage tutorial
docs/tutorial_dag.dox	Create — DAG scheduling tutorial
docs/tutorial_lockfree.dox	Create — Lock-free queue patterns tutorial
docs/faq.dox	Create — FAQ
docs/troubleshooting.dox	Create — Common issues
docs/mainpage.dox	Update — Add tutorial links
Doxyfile	Update INPUT paths

How

1. Thread Pool Selection Guide Tutorial

• When to use basic thread_pool vs typed_thread_pool
• Work-stealing configuration
• Sizing guidelines based on workload type (CPU-bound vs I/O-bound)

2. DAG Scheduling Tutorial

• Defining task dependencies
• Execution order guarantees
• Error handling in DAG chains

3. Lock-Free Queue Patterns

• MPMC vs SPSC queue selection
• Hazard pointer usage
• Performance characteristics

4. FAQ

• How many threads should I use?
• How to handle task cancellation?
• Thread pool vs std::async?
• Integration with monitoring_system

5. Troubleshooting

• Deadlock detection
• Memory leak with futures
• Platform-specific threading issues

Acceptance Criteria

• 3 tutorial pages created
• FAQ with 8+ entries
• Troubleshooting guide
• mainpage.dox updated
• Doxygen builds without warnings

Related

• Part of epic: Unify and enhance Doxygen GitHub Pages across all ecosystem systems (Phase 2) common_system#589
• After docs: Sync Doxyfile and theme to ecosystem standard #658 (Phase 1)

[kcenon]

Resolved by PR #661 (squash merged into main as 6de0642).

Phase 2 documentation enhancement delivered:

• docs/tutorial_threadpool.dox — basic vs typed pool, work-stealing, sizing for CPU- and I/O-bound workloads
• docs/tutorial_dag.dox — dependency definition, ordering guarantees, best-effort error handling
• docs/tutorial_lockfree.dox — MPMC vs SPSC selection, hazard pointers, performance characteristics
• docs/faq.dox — 10 common integration questions (threading, cancellation, monitoring, deadlocks, performance, platforms, memory, errors, testing)
• docs/troubleshooting.dox — diagnosis and fixes for deadlocks, leaks, platform issues, performance, hangs
• docs/mainpage.dox — added Learning Resources section with @ref cross-links
• Doxyfile — registered the new pages under INPUT

All CI checks passed (Ubuntu/macOS/Windows builds, sanitizers, integration tests, docs deployment).