docs(ui): add Progress Indication documentation

Add comprehensive documentation for progress indication:
- Threaded wait dialogs (IVsThreadedWaitDialog)
- IVsThreadedWaitDialog4 with async support
- Status bar progress
- Animated status bar icons
- Task Status Center for background operations
- IProgress<T> pattern integration
- Complete file processor example with all approaches
- Best practices for choosing the right mechanism

Closes #40

Author: CalvinAllen