Feat: dataflow supports markdown (#10003)

### What problem does this PR solve?

Dataflow supports markdown.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

---------

Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

rag/flow/tests/dsl_examples/general_pdf_all.json

@@ -37,9 +37,17 @@
                     "docx"
                   ],
                   "output_format": "json"
+                },
+                "markdown": {
+                  "suffix": [
+                    "md",
+                    "markdown"
+                  ],
+                  "output_format": "json"
                 }
               }
             }
+          }
         },
         "downstream": ["Chunker:0"],
         "upstream": ["Begin"]