docs: update logos for dark and light themes (#282)

- Introduce dark and light logo SVG files for better identification.
- Remove previous PNG logos to streamline assets.

Author: hwhsu1231

docs/_static/img/pyodide-logo-dark.svg

@@ -36,9 +36,14 @@
 ]
 
 html_theme = "sphinx_book_theme"
-html_logo = "_static/img/pyodide-logo.png"
 html_static_path = ["_static"]
 
+html_theme_options = {
+    "logo": {
+        "image_light": "_static/img/pyodide-logo-light.svg",
+        "image_dark": "_static/img/pyodide-logo-dark.svg",
+    },
+}
 
 sys.path.append(Path(__file__).parent.parent.as_posix())