chore: add sitemap integration and update favicon

- Add @astrojs/sitemap and configure in astro.config.mjs
- Add favicon.svg, apple-touch-icon.png, icon-192.png, icon-512.png
- Add site.webmanifest and update Layout.astro to reference icons and
  manifest
- Update robots.txt to include sitemap
- Update favicon and icon links in Layout.astro

astro.config.mjs

@@ -5,10 +5,12 @@ import { defineConfig } from "astro/config";
 import expressiveCode from "astro-expressive-code";
 import pagefind from "astro-pagefind";
 
+import sitemap from "@astrojs/sitemap";
+
 // https://astro.build/config
 export default defineConfig({
   vite: {
     plugins: [tailwindcss()],
   },
-  integrations: [expressiveCode(), pagefind()],
+  integrations: [expressiveCode(), pagefind(), sitemap()],
 });