feat: add danmuku plugin documentation and i18n setup instructions

- Created a new documentation file for the danmuku plugin with code examples.
- Added comprehensive language settings documentation, including default languages, importing languages, adding new languages, and modifying existing languages.
- Updated the i18n setup examples to reflect the latest changes in language handling.

docs/assets/ts/artplayer-plugin-multiple-subtitles.d.ts

@@ -1,8 +1,3 @@
-import type Artplayer from 'artplayer'
-
-export = artplayerPluginMultipleSubtitles
-export as namespace artplayerPluginMultipleSubtitles;
-
 declare const artplayerPluginMultipleSubtitles: (option: {
   subtitles: {
     url?: string
@@ -14,3 +9,8 @@ declare const artplayerPluginMultipleSubtitles: (option: {
 }) => (art: Artplayer) => {
   name: 'multipleSubtitles'
 }
+
+export default artplayerPluginMultipleSubtitles
+
+export = artplayerPluginMultipleSubtitles;
+export as namespace artplayerPluginMultipleSubtitles;