fix(docsearch): capture mousedown event to close modal

This doesn't close the modal when selecting the input text and finishing the selection outside of the modal.

See https://github.com/facebook/react-native-website/pull/2139#issuecomment-678330203

src/DocSearchModal.tsx

@@ -342,7 +342,7 @@ export function DocSearchModal({
       ]
         .filter(Boolean)
         .join(' ')}
-      onClick={(event) => {
+      onMouseDown={(event) => {
         if (event.target === event.currentTarget) {
           onClose();
         }