docs(agent): replace obsolete SupervisorAgent example

[March-77]

What changed

• replace the obsolete SupervisorAgent section with the current ReactAgent + AgentTool supervisor pattern
• point the code block at the maintained supervisor example in the main repository
• explain that the supervisor ReactAgent should be the A2A root while specialist agents remain internal tools
• remove the stale prompt guidance that referenced the deleted SupervisorAgent API

Why

The page currently recommends AgentTool near the top but later shows an incompatible SupervisorAgent.builder() example. Users following it encounter the missing mainAgent requirement and model-specific JSON routing failures. The current framework implements this pattern by exposing specialized agents as tools to a coordinating ReactAgent.

Fixes alibaba/spring-ai-alibaba#4514
Addresses alibaba/spring-ai-alibaba#4560
Clarifies alibaba/spring-ai-alibaba#4272

Validation

• verified the committed file by reading it back from the fork and comparing it byte-for-byte with the intended content
• verified all 15 <Code> blocks still have matching closing tags
• verified the linked implementation exists in examples/multiagent-patterns/supervisor