Improve sphinx-exercise config naming

- Change from: style_solution_after_exercise: true
- Change to: exercise_style: "solution_follow_exercise"

This makes the option name clearer and more descriptive.
Default value should be '' (empty string).

Author: mmcky

lectures/_config.yml

@@ -123,7 +123,7 @@ sphinx:
     # sphinx-proof
     proof_minimal_theme: true
     # sphinx-exercise
-    style_solution_after_exercise: true
+    exercise_style: "solution_follow_exercise"
     # sphinx-tojupyter
     tojupyter_static_file_path: ["source/_static", "_static"]
     tojupyter_target_html: true