docs: update a11y meta tags and favicon to correct values and match code-quality part

[wo-o29]

📝 Key Changes

This PR updates the meta information and favicon of the A11y Fundamentals documentation to improve accuracy and visual consistency:

• The OG title and description were previously set to values related to the Bundling Fundamentals section.

These have been corrected to:

• Title: A11y Fundamentals

• Description: Complete Guide to Frontend Accessibility

• The favicon was originally using the one from the Bundling section (/bundling/images/favicon.ico).
  It has been changed to use the favicon from the Code Quality section (/a11y/images/favicon.ico), which is defined in the shared configuration.

The favicon update aligns with the design consistency because the A11y index page’s hero image closely matches that of the Code Quality page, so sharing the same favicon provides a more coherent and unified visual experience.

🖼️ Before and After Comparison

Before	After

[vercel]

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