Issue #3557237: Add Page Template config entity and Mercury Editor integration

Steve Luongorequested to merge
issue/mercury_editor_page_templates-3557237:3557237-official-release into 1.0.x
  • Add mercury_editor_page_template config entity with admin UI and schema
  • Introduce TemplateParagraphBuilder service and hook API (api.php)
  • Refactor template selector and settings forms; update theme negotiator
  • Add example submodule with sample page template configurations
  • Add kernel and unit test coverage

Closes #3557237

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