Feat/save document converter result

[absadiki]

Hello, Thanks for the awesome package :)

This is a simple PR that adds a new save method to the DocumentConverterResult class, making it easier to save the converted document result to a file.

Changes:

• Added a save method to the DocumentConverterResult class, which takes a file_path and optional encoding parameter. The method writes the text_content to the specified file using the specified encoding.
• Updated the sample script in the README file to showcase the new save method.

[absadiki]

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