only run docs build in CI if docs have changed

Author: danielvegamyhre

.github/workflows/doc_build.yml

@@ -9,6 +9,9 @@ on:
     tags:
       - v[0-9]+.[0-9]+.[0-9]
       - v[0-9]+.[0-9]+.[0-9]+-rc[0-9]+
+    paths:
+      - 'docs/**'
+      - '!docs/**'
   pull_request:
   workflow_dispatch: