docs: update site and readme for usage-aware scanning

[sonukapoor]

Closes #214.

Summary

Updates the documentation to highlight the new usage-aware vulnerability scanning feature.

• Adds reachability capabilities to the README features list and the comparison table.
• Adds an explanation for why --usage is an opt-in flag.
• Adds a feature card and bullet point to the website (index.html).
• Updates the verbose output documentation to explain the new Usage and Breaking? columns.

Validation

• npm test
• npm run build