feat: Add IpynbConverter

- Implemented IpynbConverter class for converting Jupyter Notebook (.ipynb) files into Markdown format.
- Supports markdown cells, code cells and raw cells.
- First markdown heading is used as the title if no title is found in notebook metadata.
- Created a test notebook (`test_notebook.ipynb`) to verify the functionality of the converter.

src/markitdown/_markitdown.py

@@ -390,6 +390,63 @@ class YouTubeConverter(DocumentConverter):
                         return ret
         return None
 
+class IpynbConverter(DocumentConverter):
+    """Converts Jupyter Notebook (.ipynb) files to Markdown."""
+
+    def convert(
+        self, local_path: str, **kwargs: Any
+    ) -> Union[None, DocumentConverterResult]:
+        # Bail if not ipynb
+        extension = kwargs.get("file_extension", "")
+        if extension.lower() != ".ipynb":
+            return None
+
+        # Parse and convert the notebook
+        result = None
+        with open(local_path, "rt", encoding="utf-8") as fh:
+            notebook_content = json.load(fh)
+            result = self._convert(notebook_content)
+
+        return result
+
+    def _convert(self, notebook_content: dict) -> Union[None, DocumentConverterResult]:
+        """Helper function that converts notebook JSON content to Markdown."""
+        try:
+            md_output = []
+            title = None
+
+            for cell in notebook_content.get("cells", []):
+                cell_type = cell.get("cell_type", "")
+                source_lines = cell.get("source", [])
+
+                if cell_type == "markdown":
+                    md_output.append("".join(source_lines))
+
+                    # Extract the first # heading as title if not already found
+                    if title is None:
+                        for line in source_lines:
+                            if line.startswith("# "):
+                                title = line.lstrip("# ").strip()
+                                break
+
+                elif cell_type == "code":
+                    # Code cells are wrapped in Markdown code blocks
+                    md_output.append(f"```python\n{''.join(source_lines)}\n```")
+                elif cell_type == "raw":
+                    md_output.append(f"```\n{''.join(source_lines)}\n```")
+
+            md_text = "\n\n".join(md_output)
+
+            # Check for title in notebook metadata
+            title = notebook_content.get("metadata", {}).get("title", title)
+
+            return DocumentConverterResult(
+                title=title,
+                text_content=md_text,
+            )
+
+        except Exception as e:
+            raise FileConversionException(f"Error converting .ipynb file: {str(e)}") from e
 
 class BingSerpConverter(DocumentConverter):
     """
@@ -879,6 +936,7 @@ class MarkItDown:
         self.register_page_converter(WavConverter())
         self.register_page_converter(Mp3Converter())
         self.register_page_converter(ImageConverter())
+        self.register_page_converter(IpynbConverter())
         self.register_page_converter(PdfConverter())
 
     def convert(