adding external link (cloudflare#22925)

Author: jason-cf

src/content/docs/r2/video-tutorials.mdx

@@ -3,6 +3,7 @@ pcx_content_type: navigation
 title: Videos
 sidebar:
   order: 9
+external_link: /learning-paths/r2-intro/series/r2-1/
 ---
 
 import { CardGrid, LinkCard } from "~/components";