Small change to allow the github action to be ran without a commit

- added the line "workflow_dispatch" for a trigger in the workflow yml

.github/workflows/deploy_docs.yml

@@ -1,6 +1,7 @@
 name: publish mkdocs to github pages
 
 on:
+  workflow_dispatch:
   push:
     branches:
       - unidata_18.1.1