Improve a11y and usage for docs homepage

[HRIDYANSHU054]

Problem

The docs homepage currently exposes usability barriers that affect keyboard-only users, screen reader users, and people using touch devices.

What I changed

• Added semantic landmarks (<header>, <main>, <section>) to make page structure explicit for assistive tech.
• Added concise alt text for "Cloudinary App Gallery" images.
• Ensured clear, visible focus styles for keyboard navigation.
• Increased CTA touch targets to ~44×44px to improve tap usability on mobile.
• Honored prefers-reduced-motion to reduce motion for users who opt out.

Why this is important

• Better experience for screen reader and keyboard users — faster navigation and clearer content description.
• Improves tap accuracy on mobile for CTAs and reduces accidental misses.
• Reduces friction for contributors and reviewers by enabling the docs to run locally.

How to review / acceptance criteria

1. Pull the branch and run the docs locally: cd docs && pnpm install && pnpm dev.
2. On the homepage:
   • Use Tab to navigate and confirm focus rings are visible and contrast is high.
   • Verify gallery images have meaningful alt text (screen reader or inspect markup).
   • Confirm CTAs are at least ~44×44px (inspect element or test on a mobile device / emulation).
3. Run an automated accessibility scan (Lighthouse or axe) — high-severity issues for the homepage should be reduced.
4. Confirm the docs build locally without import resolution errors.

Files of interest

• docs/pages/index.mdx
• docs/pages/_document.js

[HRIDYANSHU054]

I have worked to solve this issue in these pr
Descriptive alt text for images in "What's inside" section
Remaining accessibility changes

@eportis-cloudinary @colbyfayock @devpatocld please review when you get a chance