feat: #3620191 Add the Search index view mode for nodes to the Varbase Content Base recipe

What changed

Adds config/core.entity_view_mode.node.search_index.yml: a disabled-by-default "Search index" view mode (node.search_index) for the Node entity type, so search integrations (e.g. Search API) can control indexed output separately from Full/Teaser instead of falling back to the Full display.

Drupal recipes auto-import every YAML file under the recipe's config/ directory, so no recipe.yml change is needed — this file is the entire change.

The view mode ships disabled (status: false), the standard Drupal pattern: it only becomes active per content type once a core.entity_view_display.node.<bundle>.search_index display is created for that bundle. Additive and non-breaking — nothing changes on an existing site until a bundle opts in. search_index is not wired into an actual search index yet; that is a follow-up.

Issue: https://www.drupal.org/project/varbase_content_base/issues/3620191

AI-Generated: Yes

Checkpoints

• 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
• Reviewed by a human
• Code review by maintainers
• Full testing and approval
• Credit contributors
• Review with the product owner
• Update Release Notes
• Release

Closes #3620191