ci: restrict docs workflow from always running

this commit updates the documentation building workflow to only run when anything in the docs directory is changed and pushed to main.

.github/workflows/build-docs.yaml

@@ -10,6 +10,8 @@ on:
   push:
     branches:
       - main
+    paths:
+      - "docs/**"
 
 env:
   SCCACHE_VERSION: v0.7.7