Enhancement: Add social icons to Footer links for better visual identity

The current footer links for social platforms (Discord, X/Twitter, Mastodon, etc.)
use a generic external-link arrow icon. While functional, this icon does not clearly
communicate the destination platform and feels slightly inconsistent with modern
documentation site design.

I suggest replacing the generic arrow icons with the official platform icons
(Discord, X/Twitter, Mastodon, etc.) next to each link.

This change would:
- Improve visual recognition of the links
- Make the footer cleaner and more visually consistent
- Better align with the branding of each platform

### Visual comparison

**Before**

**After**

I have already implemented this change locally and verified that it works as expected.
If this approach is acceptable, I can submit a Pull Request with the implementation.