feat: add danmuku plugin documentation and i18n setup guide

- Created a new documentation file for the danmuku plugin with code examples.
- Added a comprehensive guide for language settings (i18n) including default languages, importing languages, adding new languages, and modifying existing languages.
- Included code snippets for initializing the Artplayer with different language configurations.

docs/assets/ts/artplayer-plugin-iframe.d.ts

@@ -28,5 +28,5 @@ declare class ArtplayerPluginIframe {
 
 export default ArtplayerPluginIframe
 
-export = artplayerPluginIframe;
-export as namespace artplayerPluginIframe;
+export = artplayerPluginIframe
+export as namespace artplayerPluginIframe;