feat: Add the UIkit showcase and install without Layout Builder

Problem/Motivation

The demo content recipe only ships blog posts, so a fresh Website Starter install has nothing to show off the UI Suite UIkit theme's components (grids, lightbox gallery, slideshows, cards, etc.).

Installing also currently pulls in Navigation and Dashboard (via core Standard / drupal_cms_admin_ui), which need Layout Builder — a dependency this recipe should not require.

Steps to reproduce

Install the Website Starter recipe and visit the front page and admin UI: only blog content is present, and Layout Builder ends up installed as a side effect of Navigation/Dashboard.

Proposed resolution

• Add a "UIkit showcase" to the demo content: a landing page (/welcome), Features, Get started, Gallery (masonry grid + lightbox), Slideshows, Elements, Support and Image credits pages, plus 8 Web Blog posts about the theme's features. Media uses the UIkit demo images (uikit/uikit, MIT) and NASA public domain photos. Adds a "UIkit showcase" main menu dropdown and footer links; the front page stays the blog listing.
• Install without Layout Builder: stop including core Standard and drupal_cms_admin_ui as recipes (they pull in Navigation/Dashboard, which need Layout Builder); copy their needed settings into recipe.yml directly, and require the admin UI modules (gin, coffee, project_browser, automatic_updates, etc.) in composer.json instead.
• Fix the video field extra-field action to use setSettings so the demo video imports correctly.

Related: this overlaps with #3622679 (default dashboards), which needs the Dashboard module and therefore Layout Builder. Flagging it here for the maintainers to reconcile; no change made to that issue.

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

User interface changes

• New UIkit showcase pages and menu; front page unchanged (blog listing).

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• N/A