insert_document only allows Path as input

[undo76]

The current implementation of insert_document is quite limiting and doesn't support several use cases:

• Reading the documents from a CSV file
• Reading the documents from a URL

Both cases force you to save the content into a temporary file before ingestion. It is not straightforward how to allow other inputs while preserving backwards compatibility as the markdown conversion is currently done inside this method and relies on the file extension.