docs(theme): 文档补充主题配置参考链接 (#2433)

Author: visiky

docs/api/options/theme.en.md

@@ -3,4 +3,9 @@ title: Theme
 order: 9
 ---
 
-`markdown:docs/common/theme.en.md`
+`markdown:docs/common/theme.en.md`
+
+#### Refer
+
+- [G2 自定义主题](https://g2.antv.vision/en/docs/api/advanced/register-theme)
+- [G2 主题配置项详解](https://g2.antv.vision/en/docs/api/advanced/dive-into-theme)

docs/api/options/theme.zh.md

@@ -3,4 +3,9 @@ title: 图表主题
 order: 9
 ---
 
-`markdown:docs/common/theme.zh.md`
+`markdown:docs/common/theme.zh.md`
+
+#### 参阅
+
+- [G2 自定义主题](https://g2.antv.vision/zh/docs/api/advanced/register-theme)
+- [G2 主题配置项详解](https://g2.antv.vision/zh/docs/api/advanced/dive-into-theme)