Remove nav level rule (Shopify#10568)

Remove the max level rule in the Frame nav and use the `hideChildren`
boolean in frontmatter

Author: sam-b-rose

polaris.shopify.com/content/design/typography/index.md

@@ -6,6 +6,7 @@ keywords:
   - font sizes
   - fonts
 icon: TypeMajor
+hideChildren: true
 ---
 
 # {frontmatter.title}

polaris.shopify.com/src/components/Frame/Frame.tsx

@@ -14,8 +14,6 @@ import {useRouter} from 'next/router';
 import StatusBadge from '../StatusBadge';
 
 const NAV_ID = 'nav';
-// Nav levels greater than this should be part of <Subnav />
-const MAX_NAV_LEVEL = 1;
 interface Props {
   darkMode: DarkMode;
   children: React.ReactNode;
@@ -216,8 +214,7 @@ function NavItem({
 
             if (!child.slug) return null;
 
-            const isExpandable =
-              child.children && !child.hideChildren && level < MAX_NAV_LEVEL;
+            const isExpandable = child.children && !child.hideChildren;
             const id = (child.slug || key).replace(/\//g, '');
             const navAriaId = `nav-${id}`;
             const segments = asPath.slice(1).split('/');