CI: skip the native builds for docs-only pull requests

.github/workflows/ci.yml

@@ -90,7 +90,45 @@ jobs:
       - run: npm test
       - run: npm run build
 
+  changes:
+    name: Detect docs-only change
+    runs-on: ubuntu-latest
+    outputs:
+      app: ${{ steps.filter.outputs.app }}
+    steps:
+      - uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803 # v6
+        with:
+          fetch-depth: 0
+      - id: filter
+        name: Check whether anything outside site/ changed
+        shell: bash
+        env:
+          EVENT_NAME: ${{ github.event_name }}
+          BASE_REF: ${{ github.base_ref }}
+          BEFORE_SHA: ${{ github.event.before }}
+        run: |
+          if [ "$EVENT_NAME" = "pull_request" ]; then
+            base="origin/$BASE_REF"
+          else
+            base="$BEFORE_SHA"
+          fi
+          if ! files=$(git diff --name-only "$base"...HEAD 2>/dev/null); then
+            echo "Could not compare with $base; running everything."
+            echo "app=true" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+          if printf '%s\n' "$files" | grep -qvE '^site/'; then
+            echo "app=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "Only site/ changed; the native builds are skipped."
+            echo "app=false" >> "$GITHUB_OUTPUT"
+          fi
+
   native:
+    # Docs-only changes skip the native builds. A skipped job still satisfies
+    # the required status check, so such pull requests can merge.
+    needs: changes
+    if: needs.changes.outputs.app == 'true'
     strategy:
       fail-fast: false
       matrix: