docs: add iframe accessible name guidance to style guide

[Xavrir]

Resolves #8457

Changes:

Adds an "Accessible Iframe Names" section to the documentation style guide, placed after the existing "Accessible Canvas Labels" section. The new section covers:

• Using the title attribute on <iframe> elements so screen readers can identify embedded content
• A code example showing the wrong and right way to embed an iframe
• A note about using aria-hidden="true" for purely decorative iframes
• A reference to WCAG 4.1.2 (Name, Role, Value)

Also adds the new section to the table of contents.

This follows the suggestion in the parent issue (processing/p5.js-website#1115) to add a short, clear section near "Accessible Canvas Labels."

Screenshots of the change:

Not applicable — documentation-only change.

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated