Converts a document from one format to another using Pandoc. Args: input_file_path: The absolute or relative path to the input document file. output_file_path: The absolute or relative path where the converted output file should be saved. The directory will be created if it doesn't exist. to_format: The target format for the conversion (e.g., 'markdown', 'docx', 'pdf', 'html', 'rst', 'epub'). See pandoc documentation for full list. from_format: The format of the input file. If None, pandoc will try to guess from the file extension. Specify if the extension is ambiguous (e.g., 'md' for markdown). Defaults to None. extra_args: A list of additional command-line arguments to pass directly to pandoc (e.g., ['--toc'] for table of contents, ['-V', 'geometry:margin=1.5cm'] for PDF margins). Defaults to None. Returns: A string indicating success (including the output path) or an error message.