feat: page break placeholder in markdown exports options (#194)

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

docling_serve/datamodel/convert.py

@@ -296,6 +296,14 @@ class ConvertDocumentsOptions(BaseModel):
         ),
     ] = 2.0
 
+    md_page_break_placeholder: Annotated[
+        str,
+        Field(
+            description="Add this placeholder betweek pages in the markdown output.",
+            examples=["<!-- page-break -->", ""],
+        ),
+    ] = ""
+
     do_code_enrichment: Annotated[
         bool,
         Field(