docs: use a dark-mode variant of the logo

The header logo was the same SVG in both color modes; the light-colored
wordmark was hard to read on the dark theme. Point the theme's image_dark
at a new original_dark.svg whose wordmark uses light strokes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: timsaucer

docs/source/_static/images/original_dark.svg

@@ -144,7 +144,7 @@ def setup(sphinx) -> None:
     "show_toc_level": 2,
     "logo": {
         "image_light": "_static/images/original.svg",
-        "image_dark": "_static/images/original.svg",
+        "image_dark": "_static/images/original_dark.svg",
         "alt_text": "Apache DataFusion in Python",
     },
     "navbar_start": ["navbar-logo"],