Add speakernotes to the extensions list (#1355)

* Add speakernotes extension

Author: pagiraud

docs/extensions/listings/shortcodes-and-filters.yml

@@ -410,6 +410,12 @@
   description: >
     Add buttons to share articles on various social media platforms.
 
+- name: speakernotes
+  path: https://github.com/pagiraud/speakernotes
+  author: '[Pierre-Amiel Giraud](https://github.com/pagiraud/)'
+  description: >
+    Easily handle your speaker notes when exporting your presentation to other formats (mainly HTML, PDF, ODT and DOCX) to produce a handout.
+  
 - name: style-text
   path: https://github.com/leovan/quarto-style-text
   author: '[范叶亮 > Leo Van](https://github.com/leovan)'