扩展点重构

原文中：
"最终决定采用的是 JDK 标准的 SPI 扩展机制"
"并约定所有插件，都必须标注：@Extension("name")，作为加载后的标识性名称，用于配置选择"
实际上并未使用ServiceLoader，而是自己依照其思想实现了ExtensionLoader作为拓展机制，所以也无需标注@Extension("name")

Search the documentation source for the two quoted statements about JDK SPI, ServiceLoader, and @Extension. Read the surrounding page and verify it against Dubbo's ExtensionLoader implementation; update the inaccurate explanation and remove the unnecessary annotation requirement, then build the Hugo site to check the result.

Written by the indexing model from the issue text.