docs(dashboard): add admin manual to index

Updates the Web Dashboard manuals index page to include a card linking to the new "Managing Your Admin Account" manual.

Author: fulleni

src/content/docs/web-dashboard/manuals/index.mdx

@@ -16,4 +16,8 @@ This section provides user-focused manuals for administrators and publishers.
     A user manual for remotely configuring the mobile app from the web dashboard.
     [Read more →](/docs/web-dashboard/manuals/configuring-the-app/)
   </Card>
+  <Card title="Managing Your Admin Account" icon="user-cog">
+    Learn how to change the primary administrator email address.
+    [Read more &rarr;](/docs/web-dashboard/manuals/managing-admin-account/)
+  </Card>
 </CardGrid>