Redesign website (#2628)

* Bump up Bootstrap to v5.3.1, Bootstrap Icon to v1.10.5.
* Use autoComplete.js to drop dependency on jQuery and typeahead.js.
* Support dark mode.
* New svg logo and icon with responsive color mode support.
* Normalize section ids to lower kebab-case for easiness of linking.
* Use relative paths for links for local development (--root /output).
* Various markup cleanups and accessibility improvements.

docs/templates/shared/_head.html.j2

@@ -1,17 +1,23 @@
   <head>
-    <script>
-      if ((window.location.host == "jqlang.github.io") && (window.location.protocol != "https:"))
-        window.location.protocol = "https";
-    </script>
     <meta charset="utf-8">
-    <title>{{headline}}</title>
+    <title>{{ headline }}</title>
     <meta name="viewport" content="width=device-width, initial-scale=1">
-    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
-    <link rel="canonical" href="{{ url }}/{{ permalink }}" />
-    <link href="https://maxcdn.bootstrapcdn.com/bootswatch/3.3.5/spacelab/bootstrap.min.css" rel="stylesheet" integrity="sha256-j7Dtnd7ZjexEiPNbscbopFn9+Cs0b3TLipKsWAPHZIM= sha512-RFhfi6P8zWMAJrEGU+CPjuxPh3r/UUBGqQ+/o6WKPIVZmQqeOipGotH2ihRULuQ8wsMBoK15TSZqc/7VYWyuIw==" crossorigin="anonymous">
-    <link rel="stylesheet" href="{{ root }}/css/base.css" type="text/css">
-    <!--[if lt IE 9]>
-      <script src="https://oss.maxcdn.com/html5shiv/3.7.2/html5shiv.min.js"></script>
-      <script src="https://oss.maxcdn.com/respond/1.4.2/respond.min.js"></script>
-    <![endif]-->
+    <link rel="icon" href="{{ root }}/icon.svg" type="image/svg+xml">
+    <link rel="apple-touch-icon" href="{{ root }}/icon.png" type="image/png">
+    <link rel="canonical" href="{{ url }}/{{ permalink }}">
+    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootswatch@5.3.1/dist/cosmo/bootstrap.min.css"
+      integrity="sha384-dulfW0vmzZ638jigSgZXvDxMmd70GCnIv6oa+riKq6Kk4E0MKf7qmBfwP02wltv5" crossorigin="anonymous">
+    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap-icons@1.10.5/font/bootstrap-icons.css"
+      integrity="sha384-Ay26V7L8bsJTsX9Sxclnvsn+hkdiwRnrjZJXqKmkIDobPgIIWBOVguEcQQLDuhfN" crossorigin="anonymous">
+    <link rel="stylesheet" href="{{ root }}/css/style.css" type="text/css">
+    <script>
+      (function() {
+        function setTheme(mediaQuery) {
+          document.documentElement.setAttribute('data-bs-theme',
+            document.documentElement.style.colorScheme = mediaQuery.matches ? 'dark' : 'light');
+        }
+        const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
+        setTheme(mediaQuery); mediaQuery.addEventListener('change', setTheme);
+      })();
+    </script>
   </head>