add button for bill to admin/booking

pages/admin/booking/[uuid]/index.tsx

@@ -1,5 +1,7 @@
 import React, { useEffect, useState } from 'react'
 import { GetServerSideProps } from 'next'
+import { useRouter } from 'next/router'
+import Link from 'next/link'
 import Footer from '../../../../components/footer'
 import Header from '../../../../components/header'
 import Calendar from '../../../../components/calendar'
@@ -54,6 +56,7 @@ export default function ShowBookingAdmin({
 }: {
   booking: Booking
 }) {
+  const router = useRouter()
   const [booking, setBooking] = useState(bookingProp)
   const [storingBooking, setStoringBooking] = useState(false)
   const [storingBookingError, setStoringBookingError] = useState(null)
@@ -109,6 +112,9 @@ export default function ShowBookingAdmin({
           >
             Buchung Abweisen
           </button>
+          <Link href={`${router.asPath}/bill`}>
+            <a className="btn btn-gray">Rechnung</a>
+          </Link>
         </div>
       </main>