message: Add ExcelApi 1.15 reference to openpublishing config
diff: "ReferenceTOC": "docs/docs-ref-autogen/excel_1_14/toc.yml",
  "ConceptualTOCUrl": "/office/dev/add-ins/toc.json"
  },
  + {
  + "ReferenceTOC": "docs/docs-ref-autogen/excel_1_15/toc.yml",
  + "ConceptualTOCUrl": "/office/dev/add-ins/toc.json"
  + },
  {
  "ReferenceTOC": "docs/docs-ref-autogen/excel_online/toc.yml",
  "ConceptualTOCUrl": "/office/dev/add-ins/toc.json"
  