move nav bar css to nav.css,

simplify some css rules for nav responsiveness

Signed-off-by: Arthur Lu <learthurgo@gmail.com>

index.html

@@ -7,6 +7,7 @@
 		<link rel="icon" href="images/favicon.svg" sizes="any" type="image/svg+xml">
 		<link rel="stylesheet" href="https://www.w3schools.com/w3css/4/w3.css">
 		<link rel="stylesheet" href="css/style.css">
+		<link rel="stylesheet" href="css/nav.css">
 		<script src="scripts/index.js" type="module"></script>
 		<script src="scripts/instance.js" type="module"></script>
 	</head>