Prevent snapshot release when only docs push (#3527)

I "think" this is the right way to handle this to prevent unnecessary workflow runs such as this one:
https://github.com/OpenRefine/OpenRefine/actions/runs/515619980

when only a PR for docs is merged into master branch.

.github/workflows/snapshot_release.yml

@@ -4,6 +4,8 @@ on:
   push:
     branches:
       - master
+    paths:
+      - '!docs/**'
 
 jobs:
   ui_tests: