Add welcome landing page with navigation cards

[Copilot]

Implements a new welcome page as the site's landing page with greeting, branding, and navigation cards to About and Media sections.

Changes

• New landing page (index.markdown): Welcome page with gradient header, branding element, and two navigation cards (About/Media) with hover effects
• Relocated content: Previous home page → about.markdown with /about/ permalink
• Navigation update: Added "About" link to header (_includes/header.html)
• Build artifacts: Added .gitignore to exclude vendor/bundle

Design

• Responsive layout using existing Tailwind CSS patterns
• Gradient styling matches site theme (pink/orange/red → emerald/teal in dark mode)
• Navigation cards with icons, hover animations (scale + shadow)
• Accessible (decorative SVGs marked with aria-hidden)

Screenshot

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Note: _site/ is intentionally tracked for GitHub Pages deployment.

Original prompt

Implement a new welcome page for the website. This page should greet users when they visit the site and can include a simple message, branding, and navigation options. Ensure the design is visually appealing and responsive. Link the welcome page from the main navigation and set it as the initial landing page.

This pull request was created from Copilot chat.

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