Add Giscus comments integration and styling

[devnomadic]

This pull request introduces Giscus as the new commenting system for the site, replacing Disqus. It includes configuration updates, HTML template changes, and custom styling to integrate Giscus seamlessly into the site.

Giscus Integration:

• Configuration for Giscus: Added Giscus configuration in _config.yml to enable comments, including repository details, category, theme, and other settings.
• Giscus HTML Template: Created a new _includes/giscus.html file to embed the Giscus widget dynamically based on the configuration.

Replacement of Disqus:

• HTML Layout Update: Replaced the Disqus integration with Giscus in the _layouts/post.html file, ensuring comments are displayed using the new system.

Styling for Giscus:

• Custom CSS: Added styles in assets/css/custom.css to ensure proper spacing, theming, and responsiveness for Giscus comments, including support for dark mode.