add "Restrict Editing" method to public API #1641

It would be useful to support Word's document protection features, such as Restrict Editing → No Changes (Read Only).

Currently, enabling document protection requires manually modifying the underlying Open XML contained within the DOCX file. This involves extracting the document package, updating word/settings.xml to add a w:documentProtection element, and then rebuilding the DOCX archive.

While this approach works, it requires low-level knowledge of the DOCX format and direct XML manipulation. A high-level API would make it significantly easier to create documents that can be viewed and printed but are protected against accidental modification.

Possible API examples:

document.protection.read_only = True

or

document.protection.restrict_editing()

This would provide a more convenient and discoverable way to configure document protection without requiring users to work directly with Open XML internals.