Add default Header and Footer content for Varbase Starter

Problem/Motivation

The Varbase Starter site installs with an empty Canvas homepage that has no
Header or Footer content by default.

This results in a blank layout with no navigation, branding, or structural
guidance for site builders and content editors.

A starter site is expected to provide a usable baseline layout that demonstrates
the intended structure and best practices when working with Drupal Canvas
and Varbase recipes.

Proposed resolution

• Add default Header and Footer sections to the Canvas layout.
• Provide starter Canvas pages that include basic layout structure.
• Ship default Header and Footer content using recipes instead of hardcoded data.
• Ensure the default content works with the Varbase Starter installation order and Drupal Canvas logic.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ✅ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ✅ Code review from 1 Varbase core team member
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Update Release Notes
• ❌ Release varbase-11.0.0-alpha1, varbase_starter-1.0.0-alpha1

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• chore: #3569509 Add default Header and Footer content for Varbase Starter