Updated recommended CDN URL in _config.yml

[ghost]

Replace the gitalk.css link with the compressed

PR Checklist

• The commit message follows guidelines for NexT.
• Tests for the changes was maked (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs in NexT website have been added / updated (for features).

PR Type

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Build & CI related changes.
• Documentation content changes.
• Other... Please describe:

What is the current behavior?

Issue resolved: N/A

What is the new behavior?

• Screenshots with this changes: N/A
• Link to demo site with this changes: N/A

How to use?

Just updated the comment

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