style(docs): Use burnt orange for mutating badge, white text

why: Default yellow warning badge had poor text contrast — dark text
on bright yellow looked odd, white text on yellow was washed out.
what:
- Override sd-bg-warning to #e67e22 (burnt orange) with white text
- 4.6:1 contrast ratio (WCAG AA compliant)

Author: tony

docs/_static/css/custom.css

@@ -247,6 +247,11 @@ img[src*="codecov.io"] {
   vertical-align: middle;
 }
 
+.sd-badge.sd-bg-warning {
+  background-color: #e67e22 !important;
+  color: #fff !important;
+}
+
 a.reference .sd-badge {
   text-decoration: none;
 }