Improvements to the copyright term page: term description

This update adds a short explanatory paragraph to the copyright term pages (50 years pma and 70 years pma). The description is dynamically fetched from the corresponding Wikidata item and displayed between the page title and the country grid.

The paragraph styling matches the existing one on the Countries and Terms page for design consistency.

These in-turn:

Improves understanding of copyright term meanings.

Enhances visual and informational consistency across pages.