[Bug] Tag Style Overrides Fix for Pre/Code tags

[dfop02]

Description

This PR fixes the tag_style_ovrrides for code/pre tags, that was not being applied correctly.
This PR also improves the documentation around custom Word style usage when working with template-based documents.

It clarifies that:

• Custom styles must already exist in the .docx document being generated.
• The same Document instance loaded from the template must be passed to the parser and saved.
• Creating and saving a new Document() will result in losing template-defined custom styles.
• A warning is logged when a referenced custom style does not exist in the document at generation time.

Issue Reference

Fixes #62

Checklist Before Requesting a Review

• I have performed a self-review of my code.
• My code follows the project's coding style and guidelines.
• I have run tests and verified that all existing and new tests pass.
• I have added new tests to cover my changes.