docs: add typings to some examples

README.md

@@ -33,7 +33,7 @@ import { DayPicker } from "react-day-picker";
 import "react-day-picker/dist/style.css";
 
 function MyDatePicker() {
-  const [selected, setSelected] = useState();
+  const [selected, setSelected] = useState<Date | undefined>();
   return <DayPicker mode="single" selected={selected} onSelect={setSelected} />;
 }
 ```

website/docs/start.mdx

@@ -30,7 +30,7 @@ import { DayPicker } from "react-day-picker";
 import "react-day-picker/style.css";
 
 export function MyDatePicker() {
-  const [selected, setSelected] = useState();
+  const [selected, setSelected] = useState<Date | undefined>();
   return <DayPicker mode="single" selected={selected} onSelect={setSelected} />;
 }
 ```

website/docs/using-daypicker/selection-modes.mdx

@@ -217,7 +217,7 @@ import { useState } from "react";
 import { DayPicker } from "react-day-picker";
 
 export function App() {
-  const [selected, setSelected] = useState();
+  const [selected, setSelected] = useState<Date[] | undefined>();
   const handleSelect = (newSelected) => {
     // Update the selected dates
     setSelected(newSelected);