Update shortcodes-and-filters.yml (#1469)

Added description of new short code extension `si`

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

@@ -427,6 +427,12 @@
   description: >
     This extension lets you embed [Shinylive](https://shiny.rstudio.com/py/docs/shinylive.html) applications in a Quarto document.
 
+- name: si
+  path: https://github.com/GeorgRamer/si
+  author: '[Georg Ramer](https://github.com/GeorgRamer)'
+  description: > 
+    This extension creates a shortcode si for easy type setting SI values and units. It should be agnostic of output formats. 
+
 - name: social-embeds
   path: https://github.com/sellorm/quarto-social-embeds
   author: '[sellorm](https://github.com/sellorm)'