docs: adopt brand-kit favicon and header mark (#48)

Swap the docs site onto the canonical modern-python brand kit:
- favicon -> assets/favicon.svg (the pinwheel mark)
- header logo -> assets/mark.svg (chevron mark), leaving the project's own
  site title beside it instead of the org "MODERN PYTHON" wordmark
- drop the superseded modern-python-white.svg / modern-python-mark.svg

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

Author: lesnik512

docs/assets/favicon.svg

@@ -14,8 +14,8 @@ nav:
       - Testing: usage/testing.md
 theme:
   name: material
-  logo: assets/modern-python-white.svg
-  favicon: assets/modern-python-mark.svg
+  logo: assets/mark.svg
+  favicon: assets/favicon.svg
   features:
     - content.code.copy
     - content.code.annotate