[Fix Accessibility] Add focus-visible styles to Mobile Table of Contents component

Author: jpg619

src/components/table-of-contents.js

@@ -53,6 +53,10 @@ export const Mobile = withTableOfContents(({items}) => {
             borderBottomWidth: open ? 1 : 0,
             borderBottomLeftRadius: open ? 0 : 2,
             borderBottomRightRadius: open ? 0 : 2,
+            '&:focus-visible': {
+              outline: '2px solid',
+              outlineColor: '-webkit-focus-ring-color',
+            },
           }}
           leadingIcon={open ? ChevronDownIcon : ChevronRightIcon}
         >