Docs: add Symfony Console features documentation

- Add invokable commands with #[Argument] and #[Option] attributes
- Add shell completion setup guide for Bash, Zsh, Fish
- Add signal handling for graceful shutdowns
- Add SymfonyStyle usage guide with prompts and progress bars
- Add console events integration with Nette
- Add Cursor helper for terminal control
- Add TreeHelper for hierarchical data display
- Add command testing guide with CommandTester
- Reorganize README with logical sections (Getting started, Commands, UI, Advanced, Testing)
- Add links to official Symfony documentation

Author: felixure