Improve documentation and update examples

[f3l1x]

• Add structured Table of Contents with nested sections
• Document Messages as simple POPOs
• Document Handlers with attributes and Neon tags examples
• Document multiple handlers in one class
• Document all three bus types (MessageBus, CommandBus, QueryBus)
• Document Bus registry and custom bus configuration
• Document all transport types (sync, in-memory, Redis, Doctrine, AMQP)
• Document routing options (basic, interface, wildcard)
• Document all console commands with options and examples
• Document advanced topics: retry strategy, failure transport, middleware
• Document batch handlers and event listeners
• Document serializers and logging configuration
• Add Testing section with in-memory transport and BufferLogger
• Add full configuration example
• Link to Symfony documentation where relevant
• Use callout boxes for important notes