[docs] Build docs only when doc files change (#812)

Build docs only when change happens in `docs/source`

See for example
https://github.com/huggingface/api-inference/blob/main/.github/workflows/build_documentation.yml#L3-L8

.github/workflows/build_documentation.yml

@@ -2,6 +2,8 @@ name: Build documentation
 
 on:
   push:
+    paths:
+      - "docs/source/**"
     branches:
       - main
       - doc-builder*

.github/workflows/build_pr_documentation.yml

@@ -2,6 +2,8 @@ name: Build PR Documentation
 
 on:
   pull_request:
+    paths:
+      - "docs/source/**"
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}