refactor: extract redundant max-width into utility (#525)

* refactor: extract redundant max-width into utility

Extract redundant max-width into a universal tailwind
utility class. By doing so, the width of the layout
in different pages can be customized easily.

* docs: update docs for modifying `max-w-app` utility

src/components/BackButton.astro

@@ -6,7 +6,7 @@ import { SITE } from "@/config";
 
 {
   SITE.showBackButton && (
-    <div class="mx-auto flex w-full max-w-3xl items-center justify-start px-2">
+    <div class="mx-auto flex w-full max-w-app items-center justify-start px-2">
       <LinkButton
         id="back-button"
         href="/"