Update deploy.yml

Allow manual runs of the GH pages action

Signed-off-by: Russell Trow <russell@greensoftware.foundation>

Author: russelltrow

.github/workflows/deploy.yml

@@ -1,6 +1,7 @@
 name: Deploy to GitHub Pages
 
 on:
+  workflow_dispatch:
   push:
     branches:
       - main