Fix accessibility violations: Add alt text to citation images

[huangkevin-apr]

Summary

This PR resolves 3 accessibility violations by adding descriptive alt text to citation images in the experts section.

Problem

The IBM Equal Access Accessibility Checker identified that three <img> elements were missing alternative text, violating WCAG Level A guidelines. These images are academic paper citations that link to research publications but were not accessible to screen reader users.

• Violation: Hyperlinks must have an accessible name for their purpose
• Impact: Users relying on assistive technologies could not understand the purpose of these linked citation images.

Solution

Added descriptive alt attributes to all three citation images:

• Let's Encrypt paper (aas.png): Alt text describes the paper title and topic
• HTTPS Usability paper (krombholz.png): Alt text includes the quoted paper title
• TLS Crypto Shortcuts paper (springall.png): Alt text provides the paper title

Changes

• Added meaningful alt text to 3 <img> elements in src/index.html
• Alt text reflects the academic paper titles/topics being cited
• Maintains semantic HTML structure without breaking existing layout

Testing

✅ All 3 accessibility violations resolved
✅ Images remain properly linked to DOI/conference URLs
✅ Screen readers can now announce meaningful context for each citation
✅ Visual presentation unchanged