only run docs CI jobs on PRs when docs have changed (#1612)

only run docs CI jobs when docs have changed

.github/workflows/doc_build.yml

@@ -9,10 +9,10 @@ on:
     tags:
       - v[0-9]+.[0-9]+.[0-9]
       - v[0-9]+.[0-9]+.[0-9]+-rc[0-9]+
+  pull_request:
     paths:
       - 'docs/**'
       - '!docs/**'
-  pull_request:
   workflow_dispatch:
 
 concurrency: