🎨 (basic): Organiza estilos de Basic e Intro

Divide `basic.css` en parciales SCSS bajo `assets/scss/basic/`, integra
`normalize.css` en `basic.min.css` y compila `intro.scss` por separado a
`intro.min.css`.

assets/scss/basic/_nav.scss

@@ -0,0 +1,61 @@
+.nav {
+    display: flex;
+    align-items: center;
+    gap: 0.25rem;
+    list-style: none;
+    margin: 0;
+    padding: 0;
+}
+
+.nav-item {
+    position: relative;
+}
+
+.nav-link {
+    display: block;
+    padding: 0.5rem 0.75rem;
+    color: var(--val-color--text);
+    text-decoration: none;
+    border-radius: 0.375rem;
+}
+.nav-link:hover {
+    background-color: color-mix(in srgb, var(--val-color--text) 8%, transparent);
+}
+.nav-link.active {
+    color: var(--val-color--primary);
+    font-weight: 600;
+}
+.nav-link.disabled {
+    color: var(--val-color--text--muted);
+    pointer-events: none;
+}
+// The toggle of a dropdown inside a `Nav` looks like the other links: it drops the border that a
+// standalone `Dropdown` toggle has (see `_buttons.scss`).
+.nav-link.dropdown-toggle {
+    width: 100%;
+    text-align: start;
+    border: 0;
+}
+
+.nav-start {
+    justify-content: flex-start;
+}
+.nav-center {
+    justify-content: center;
+}
+.nav-end {
+    justify-content: flex-end;
+}
+.nav-vertical {
+    flex-direction: column;
+    align-items: stretch;
+}
+.nav-fill > .nav-item {
+    flex: 1 1 auto;
+    text-align: center;
+}
+.nav-justified > .nav-item {
+    flex-basis: 0;
+    flex-grow: 1;
+    text-align: center;
+}