fix: Remove unsupported book.toml keys

Removed configuration keys from book.toml not supported by the current
version of mdbook, which were causing build warnings.

theme/navbar.js

@@ -0,0 +1,12 @@
+document.addEventListener('DOMContentLoaded', function () {
+    var navbar = document.createElement('nav');
+    navbar.className = 'navbar';
+    navbar.innerHTML =
+        '<div class="nav-container">' +
+            '<a href="/" class="nav-brand">Home</a>' +
+            '<ul class="nav-menu">' +
+                '<li><a href="https://forgejo.binning.net" target="_blank">Git</a></li>' +
+            '</ul>' +
+        '</div>';
+    document.body.insertBefore(navbar, document.body.firstChild);
+});