docs: add favicon matching the main datafusion site

Drop in the same favicon.svg the main datafusion.apache.org site
uses (just the Apache DataFusion mark, no wordmark) and wire it
through html_favicon. Browsers and bookmarks now show the project
icon instead of the generic Sphinx page glyph.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

docs/source/_static/favicon.svg

@@ -156,6 +156,8 @@ def setup(sphinx) -> None:
 # so a file named "default.css" will overwrite the builtin "default.css".
 html_static_path = ["_static"]
 
+html_favicon = "_static/favicon.svg"
+
 # Copy agent-facing files (llms.txt) verbatim to the site root so they
 # resolve at conventional URLs like `https://.../python/llms.txt`.
 html_extra_path = ["llms.txt"]