fix needs: groups, add changed-files guard for docs build

Author: jameslamb

.github/workflows/build.yaml

@@ -267,6 +267,7 @@ jobs:
       build_type: ${{ inputs.build_type || 'branch' }}
       matrix_name: docs-build
   docs-build:
+    needs: [docs-build-matrix]
     if: github.ref_type == 'branch'
     permissions:
       actions: read

.github/workflows/pr.yaml

@@ -743,9 +743,10 @@ jobs:
       contents: read
     uses: rapidsai/shared-workflows/.github/workflows/compute-matrix.yaml@release/26.10
     with:
-      build_type: ${{ inputs.build_type || 'branch' }}
+      build_type: pull-request
       matrix_name: docs-build
   docs-build:
+    needs: [docs-build-matrix]
     permissions:
       actions: read
       contents: read
@@ -754,6 +755,7 @@ jobs:
       pull-requests: read
     secrets: inherit # zizmor: ignore[secrets-inherit]
     uses: rapidsai/shared-workflows/.github/workflows/custom-job.yaml@release/26.10
+    if: fromJSON(needs.changed-files.outputs.changed_file_groups).build_docs
     strategy:
       fail-fast: false
       matrix: ${{ fromJSON(needs.docs-build-matrix.outputs.matrix) }}