docs: add marked-plaintify to known extensions (#3265)

docs/USING_ADVANCED.md

@@ -96,6 +96,7 @@ Marked can be extended using [custom extensions](/using_pro#extensions). This is
 |[LinkifyIt](https://github.com/UziTech/marked-linkify-it)|[`marked-linkify-it`](https://www.npmjs.com/package/marked-linkify-it)|Use [linkify-it](https://github.com/markdown-it/linkify-it) for urls|
 |[Mangle](https://github.com/markedjs/marked-mangle)|[`marked-mangle`](https://www.npmjs.com/package/marked-mangle)|Mangle mailto links with HTML character references|
 |[Misskey-flavored Markdown](https://akkoma.dev/sfr/marked-mfm)|[`marked-mfm`](https://www.npmjs.com/package/marked-mfm)|Custom extension for [Misskey-flavored Markdown](https://github.com/misskey-dev/mfm.js/blob/develop/docs/syntax.md).|
+|[Plaintify](https://github.com/bent10/marked-extensions/tree/main/packages/plaintify)|[`marked-plaintify`](https://www.npmjs.com/package/marked-plaintify)|Converts Markdown to Plaintext|
 |[Shiki](https://github.com/bent10/marked-extensions/tree/main/packages/shiki)|[`marked-shiki`](https://www.npmjs.com/package/marked-shiki)|Integrating [Shiki](https://shiki.style/) syntax highlighting|
 |[Sequential Hooks](https://github.com/bent10/marked-extensions/tree/main/packages/sequential-hooks)|[`marked-sequential-hooks`](https://www.npmjs.com/package/marked-sequential-hooks)|Enables the sequential preprocessing and post-processing within [sequential hooks](https://github.com/bent10/marked-extensions#sequential-hooks)|
 |[Smartypants](https://github.com/markedjs/marked-smartypants)|[`marked-smartypants`](https://www.npmjs.com/package/marked-smartypants)|Use [smartypants](https://www.npmjs.com/package/smartypants) to use "smart" typographic punctuation for things like quotes and dashes.|