doc: ignore panel css to increase main content width

Otherwise the main content ends up being weirdly small
See:
https://github.com/executablebooks/sphinx-book-theme/blob/547cc95674623345124d9bbea9e15e5b5fa34d0e/docs/conf.py#L62
https://sphinx-panels.readthedocs.io/en/latest/index.html?highlight=panels_add_bootstrap_css#sphinx-configuration

docs/conf.py

@@ -121,6 +121,9 @@
     "python": ("https://docs.python.org/3", None),
 }
 
+# remove panels css to get wider main content
+panels_add_bootstrap_css = False
+
 # Settings for copybutton
 copybutton_prompt_is_regexp = True
 copybutton_prompt_text = r">>> |\.\.\. "  # doctest