docs: fix broken English links and complete showcases section

[eduardozaniboni]

Problem

The English documentation home page (docs/en-US/index.md) was missing link properties in several frontmatter items within the concepts and showcases sections. This caused VitePress to generate broken links like el_USundefined.html when rendering the home page components.
Additonally, the English "Showcases" section was incomplete compared to the Portuguese and Spanish versions, lacking serveral examples (Material UI, Chakra UI, Vue, etc.).

Solution

1. Frontmatter Fix: Added all missing link properties to docs/en-US/index.md.
2. Content Completition: Added the missing showcase items to the Ensligh home page to ensure consistency across all supported languages.
3. Defensive UI: Updated ConceptsSection.vue to include a URL fallback (#) if a link property is missing, preventing complete URL breakage in the future.

Changes

• docs/en-US/index.md: Added links and missing showcase items.
• docs/components/home/ConceptsSection.vue: Added URL fallback logic.

Verification

• Verified that all links in the English home page now point to the correct documentation pages.
• Verified that the "Showcases" section now correctly displays all 5 examples.
• Verified that no regressions were introduced to Portuguese or Spanish translations.