Apply suggestion from @yaswant

Co-authored-by: Yaswant Pradhan <2984440+yaswant@users.noreply.github.com>

Author: Pierre-siddall

.github/workflows/build-sphinx-docs.yaml

@@ -60,7 +60,12 @@ jobs:
     steps:
       - name: Deprecation warning
         run: |
-          echo "::warning title=Deprecated workflow::This reusable workflow is deprecated and will be removed in the future. Please migrate to the sphinx-docs.yaml workflow."
+          echo "::warning title=Workflow Deprecated::This reusable workflow will be removed in future release. Please migrate to sphinx-docs.yaml workflow."
+
+          echo "### ⚠️ Deprecation Notice" >> $GITHUB_STEP_SUMMARY
+echo "The \`build-sphinx-docs\` reusable workflow has been **deprecated** and will be removed in the next release." >> $GITHUB_STEP_SUMMARY
+          echo "" >> $GITHUB_STEP_SUMMARY
+          echo "> **Migration Path:** Please transition your workflows to use the new unified action instead. Refer to the [sphinx-docs workflow guide](https://github.com/MetOffice/growss/tree/main/sphinx-docs) for details." >> $GITHUB_STEP_SUMMARY
 
       - name: Checkout repository
         uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1