docs(examples): update v2 example class references (#2209)

## Description

Update the examples README to match the current AgentScope Java 2.0
source structure.

Changes include:

- Update example main class package names to
`io.agentscope.examples.documentation2`.
- Replace the removed `HookExample` with `CustomizedMiddlewareExample`.
- Replace the renamed `SessionExample` with `StateExample`.
- Replace the former combined `McpToolExample` with the current
transport-specific examples:
  - `McpStdioExample`
  - `McpSseExample`
  - `McpStreamableHttpExample`
- Document the environment variables required by the SSE and Streamable
HTTP examples.