workflows: doc: skip preview when doc is not modified on PR (#233)

Signed-off-by: Julien Stephan <jstephan@baylibre.com>

Author: justeph

.github/workflows/docs.yml

@@ -10,6 +10,8 @@ on:
       - "docs/**"
   pull_request:
     types: [opened, synchronize, reopened, closed]
+    paths:
+      - "docs/**"
   workflow_dispatch:
 
 concurrency: