chore(theme): swap header DZ icon to Color Black variant (#184)

The header was using the Dark Mode variant of the DZ icon (light-gray
outlines), which reads washed-out against the slate header. Replace it
with the official Color Black PNG, matching the icon on doublezero.xyz.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: Jared-dz

docs/dz-icon.png

@@ -1,2 +1,2 @@
-<img src="{{ 'logo.svg' | url }}" alt="DoubleZero" />
+<img src="{{ 'dz-icon.png' | url }}" alt="DoubleZero" />
 <img src="{{ config.theme.logo | url }}" alt="{{ config.site_name }}" />