fix: #3591967 Improve page variant backward compatibility for recipes

• Ensure the Page content marker component is created even when the recipe doesn't provide it.
• Run automatic migration to page variant after site template (recipe with type: Site) is installed.
• Render existing page regions when page variant couldn't be resolved and there are enabled regions for the active theme.

Testing instructions

• checkout canvas 1.x
• install site template (for example Byte)
  • observe that global regions are not displayed and no page variants were automatically created
• install site from a recipe of type different than 'Site'
  • observe that global regions are not displayed and no page variants were automatically created
• checkout changes from this MR
• install site template
  • verify that page regions provided by the recipes were migrated to page variant and that page chrome like site header/footer is displayed
• install site from a recipe of type different than 'Site'
  • verify that global regions are rendered and page variant wasn't automatically created
  • verify that after creating a new page variant manually and setting it as default it is rendered instead of global regions

Closes #3591967 (closed)

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AI-Generated: Yes (Used AI coding agent to work on the changes).