Add nested Block Grid example for test site

[brianlyktwoday]

Description

This PR adds a sample nested Block Grid layout to the Umbraco test site so we can validate hidden-item filtering in nested grid structures.

What’s included:

• Adds a two-column row component for Block Grid items
• Renders nested Block Grid areas via the component partial
• Adds the CSS needed to style and layout nested grid items correctly
• Updates the seeded SQLite database with the required document types and data type configuration so the example works out of the box

[andreaslborg]

Looks solid and works on v18. Thanks again @brianlyktwoday 🥳