Fix heading structure for improved accessibility

[AshmitSherigar]

This pull request addresses heading level inconsistencies across several pages in the p5.js website to improve semantic HTML structure and accessibility.

Changes made:

• Reviewed all major pages listed in the site navigation and footer.
• Ensured each page has a single h1 element followed by properly nested h2 and h3 where applicable.
• Specifically corrected heading levels on the following pages:
  • / (home page): Adjusted heading levels so h1 precedes all h2s.
  • /download: Updated h3 to h2 for proper hierarchy.

Why this matters:

• Ensures semantic correctness for screen readers.
• Improves accessibility and SEO.
• Aligns with WCAG best practices and HTML5 standards.

Let me know if any additional pages need updating.

Fixes #872