wip: book documentation

docs/src/concepts/04-hooks.md

@@ -0,0 +1,16 @@
+```rust
+fn Example<'a>(cx: Context<'a>, props: &()) -> DomTree<'a> {
+    let service = use_combubulator(cx);
+    let Status { name, pending, count } = service.info();
+    html! {
+        <div>
+            <p> "Hello, {name}!" </p>
+            <p> "Status: {pending}!" </p>
+            <p> "Count {count}!" </p>
+            <button onclick={|_| service.update()}>
+                "Refresh services"
+            </button>
+        </div>
+    }
+}
+```