.github/workflows: enable manual dispatch for some workflows (#5929)

.github/workflows/docs.yml

@@ -1,7 +1,8 @@
 name: Documentation
-# This job builds and deploys documenation to github pages.
-# It runs on every push to master.
+# This job builds and deploys documentation to github pages.
+# It runs on every push to master, and can be manually triggered.
 on:
+  workflow_dispatch: # allow running workflow manually
   push:
     branches:
       - master