Add Confluence video how-to embed link (#340)

Author: Paul-Cornell

snippets/general-shared-text/confluence.mdx

@@ -1,5 +1,15 @@
 The Confluence prerequisites:
 
+<iframe
+width="560"
+height="315"
+src="https://www.youtube.com/embed/tXu6S6Pd7Dc"
+title="YouTube video player"
+frameborder="0"
+allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture"
+allowfullscreen
+></iframe>
+
 - A [Confluence account](https://www.atlassian.com/software/confluence).
 - The URL for the target Confluence instance.
 - A [Confluence user](https://confluence.atlassian.com/doc/add-and-invite-users-138313.html) with