docs: remove baked-in canvas from dark-mode logo

The dark logo SVG is an auto-trace that carries its background as nine
opaque fill paths (#212529), invisible on the old dark-gray header but
visible as a box on the navy redesign. Delete the background paths and
recolor the seven letter counters and pixel notches to the navy surface
color so the wordmark renders clean in the header and footer.

Same fix as applied to the trust-center copy of this logo; the SVG is
still not truly transparent inside the letter counters, so reusing it on
non-navy backgrounds would need a proper even-odd hole merge.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: 2 people