Add sidebar to landing page for mobile navigation

Co-authored-by: Ona <no-reply@ona.com>

Author: Siddhant-K-code

src/pages/index.astro

@@ -1,6 +1,7 @@
 ---
 import BaseLayout from "../layouts/BaseLayout.astro";
 import Header from "../components/Header.astro";
+import Sidebar from "../components/Sidebar.astro";
 import Footer from "../components/Footer.astro";
 import { getCollection } from "astro:content";
 import { groupByPart, type ChapterMeta } from "../lib/chapters";
@@ -21,6 +22,7 @@ const totalChapters = allChapters.filter((c) => c.chapter).length;
 
 <BaseLayout title="The Agentic Engineering Guide | AI Agent Infrastructure Book">
   <Header />
+  <Sidebar chapters={allChapters} currentSlug="" />
 
   <main class="pt-14">
     <!-- Hero -->