refactor: move Excel markdown serializer to docling-core

Remove `docling/utils/markdown.py` and update imports to use
`docling_core.transforms.serializer.markdown_excel` as requested in
code review on #3255. The serializer now lives in docling-core#587.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Smeet23

docling/utils/markdown.py

@@ -59,7 +59,9 @@ def documents() -> list[tuple[Path, DoclingDocument]]:
 
 
 def test_e2e_excel_conversions(documents) -> None:
-    from docling.utils.markdown import MsExcelMarkdownDocSerializer
+    from docling_core.transforms.serializer.markdown_excel import (
+        MsExcelMarkdownDocSerializer,
+    )
 
     for gt_path, doc in documents:
         pred_md: str = MsExcelMarkdownDocSerializer(doc=doc).serialize().text
@@ -282,7 +284,9 @@ def test_sheet_names_as_headings(documents) -> None:
     injected into the document model by the backend.
     Sheet4 in xlsx_01 is empty and intentionally has no heading.
     """
-    from docling.utils.markdown import MsExcelMarkdownDocSerializer
+    from docling_core.transforms.serializer.markdown_excel import (
+        MsExcelMarkdownDocSerializer,
+    )
 
     doc = next(item for path, item in documents if path.stem == "xlsx_01")