docs: add marked-footnote to known extensions (#3034)

docs/USING_ADVANCED.md

@@ -87,6 +87,7 @@ Marked can be extended using [custom extensions](/using_pro#extensions). This is
 |[Custom Heading ID](https://github.com/markedjs/marked-custom-heading-id)|[`marked-custom-heading-id`](https://www.npmjs.com/package/marked-custom-heading-id)|Specify a custom heading id in headings with the [Markdown Extended Syntax](https://www.markdownguide.org/extended-syntax/#heading-ids) `# heading {#custom-id}`|
 |[Emoji](https://github.com/UziTech/marked-emoji)|[`marked-emoji`](https://www.npmjs.com/package/marked-emoji)|Add emoji support like on GitHub|
 |[Extended Tables](https://github.com/calculuschild/marked-extended-tables)|[`marked-extended-tables`](https://www.npmjs.com/package/marked-extended-tables)|Extends the standard Github-Flavored tables to support advanced features: Column Spanning, Row Spanning, Multi-row headers|
+|[Footnote](https://github.com/bent10/marked-extensions/tree/main/packages/footnote)|[`marked-footnote`](https://www.npmjs.com/package/marked-footnote)|Enables [GFM footnotes](https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#footnotes)|
 |[GFM Heading ID](https://github.com/markedjs/marked-gfm-heading-id)|[`marked-gfm-heading-id`](https://www.npmjs.com/package/marked-gfm-heading-id)|Use [`github-slugger`](https://github.com/Flet/github-slugger) to create the heading IDs and allow a custom prefix.|
 |[Highlight](https://github.com/markedjs/marked-highlight)|[`marked-highlight`](https://www.npmjs.com/package/marked-highlight)|Highlight code blocks|
 |[Katex Code](https://github.com/UziTech/marked-katex-extension)|[`marked-katex-extension`](https://www.npmjs.com/package/marked-katex-extension)|Render [katex](https://katex.org/) code|