docs: customization chapter exploration (#9078)

Adds a new customizations chapter with realistic example while maintaining the linear learning journey.

Preview: https://docs-v2-git-docs-customizations-chapter-medusajs.vercel.app/v2/customization

www/packages/docs-ui/src/components/Note/Layout/index.tsx

@@ -75,7 +75,7 @@ export const NoteLayout = ({ type, title, children }: NoteLayoutProps) => {
   return (
     <div
       className={clsx(
-        "py-[10px] px-docs_0.75 my-docs_0.5",
+        "py-[10px] px-docs_0.75 my-docs_1",
         "flex gap-docs_0.75 rounded-docs_DEFAULT items-stretch",
         "bg-medusa-bg-component border border-medusa-border-base"
       )}