feat: rewrite to use app config and rework docs (#143)

Co-authored-by: Daniel Roe <daniel@roe.dev>
Co-authored-by: Sébastien Chopin <seb@nuxt.com>

docs/composables/useScrollspy.ts

@@ -0,0 +1,37 @@
+/**
+ * Scrollspy allows you to watch visible headings in a specific page.
+ * Useful for table of contents live style updates.
+ */
+export const useScrollspy = () => {
+  const observer = ref() as Ref<IntersectionObserver>
+  const visibleHeadings = ref([]) as Ref<string[]>
+  const activeHeadings = ref([]) as Ref<string[]>
+
+  const observerCallback = (entries: IntersectionObserverEntry[]) =>
+    entries.forEach((entry) => {
+      const id = entry.target.id
+
+      if (entry.isIntersecting) { visibleHeadings.value.push(id) } else { visibleHeadings.value = visibleHeadings.value.filter(t => t !== id) }
+    })
+
+  const updateHeadings = (headings: Element[]) =>
+    headings.forEach((heading) => {
+      observer.value.observe(heading)
+    })
+
+  watch(visibleHeadings, (val, oldVal) => {
+    if (val.length === 0) { activeHeadings.value = oldVal } else { activeHeadings.value = val }
+  })
+
+  // Create intersection observer
+  onBeforeMount(() => (observer.value = new IntersectionObserver(observerCallback)))
+
+  // Destroy it
+  onBeforeUnmount(() => observer.value?.disconnect())
+
+  return {
+    visibleHeadings,
+    activeHeadings,
+    updateHeadings
+  }
+}