Enable minimal theme for proof/admonition directives (#334)

Set proof_minimal_theme: true to match lecture-python.myst styling.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: mmcky

lectures/_config.yml

@@ -128,6 +128,8 @@ sphinx:
           "gG": "\\mathcal{G}"
           "hH": "\\mathcal{H}"
     mathjax_path: https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js
+    # sphinx-proof
+    proof_minimal_theme: true
     # sphinx-exercise
     exercise_style: "solution_follow_exercise"
     rediraffe_redirects: