docs: fix the APISIX Dubbo gateway sample

[Yilialinn]

What is the purpose of this change?

The APISIX Dubbo gateway sample README currently uses an incorrect service class name, mixes an old method name into the explanation, contains a non-executable Markdown-formatted cURL URL, and embeds an Admin API key. It also omits the complete startup and verification order needed to run the sample.

What does this change do?

• Aligns the service FQCN and method with ApisixService and ApisixServiceImpl.
• Reads the APISIX Admin API key from the local configuration.
• Documents the Nacos, APISIX, Provider, Upstream, Route, request, and cleanup sequence.
• Explains the Hessian2 and Map<String, Object> contract used by dubbo-proxy.
• Separates this HTTP-to-Dubbo example from the existing Dubbo 3 Triple sample.
• Replaces the stale browser-header output with the response actually returned by the current sample.

How was it verified?

• Built the selected Provider reactor with Maven 3.9.11 on JDK 17.
• Ran Apache APISIX 3.17.0, etcd 3.5.11, Nacos 2.1.1, and the Dubbo 3.3.1 Provider on an isolated Docker network.
• Confirmed Nacos registered a healthy gateway-apisix-dubbo instance on port 20880.
• Created the documented Upstream and Route successfully.
• Sent the documented JSON request and observed HTTP 200, author: yang siming, and dubbo success.
• Repeated the Provider startup with the README's exact JRE 17 bind-mount command.
• Parsed the shell, JSON, and YAML snippets and checked all links.

Scope

This PR changes only the existing APISIX Dubbo sample README. It does not change Java, POM files, dependency versions, the Triple sample, Docker Compose files, or the Dubbo website article.