docs: add docs for plugin ExtensionGetter (#388)

#### What this PR does / why we need it:

添加 ExtensionGetter 的开发者文档及示例。

see https://github.com/halo-dev/halo/pull/6365

#### Does this PR introduce a user-facing change?
```release-note
None
```

sidebars.js

@@ -185,6 +185,7 @@ module.exports = {
                 "developer-guide/plugin/api-reference/server/template-for-theme",
                 "developer-guide/plugin/api-reference/server/websocket",
                 "developer-guide/plugin/api-reference/server/login-handler-enhancer",
+                "developer-guide/plugin/api-reference/server/extension-getter",
                 {
                   type: "category",
                   label: "扩展点",