#3951 docs: fix broken Python SDK link

Fixes the broken Python SDK link in the README.

The previous link pointed to: https://github.com/superdoc/docx-editor/tree/main/packages/sdk/langs/python

That path no longer exists.

This change updates it to the current SuperDoc Python SDK documentation: https://docs.superdoc.dev/agents/automation/python-sdk/

The new link points users to the maintained Python SDK installation and usage documentation.