chore(design-system): better breadcrumbs (#42310)

## What kind of change does this PR introduce?

Minor update

## What is the current behavior?

Our design system’s breadcrumbs:

1. Are not interactive
2. ~~Repeat the page title, which is directly below~~

## What is the new behavior?

These breadcrumbs:

- Are hyperlinked
- Show all parents, if nested
- ~~Don’t repeat the page title~~

| Before | After |
| --- | --- |
| <img width="1024" height="756"
alt="Forms-FBDF180F-F101-407C-A8EB-A06BBDB3509A"
src="https://github.com/user-attachments/assets/f2592b8b-b5d4-415f-af2e-28b99d5de0a0"
/> | <img width="1024" height="794"
alt="Forms-95348190-C8B2-4A3A-AE55-A4A5A16CD4B4"
src="https://github.com/user-attachments/assets/7b296c20-b12a-48ff-9479-6547cda73cd5"
/> |

## Additional context

I wonder if we should either drop the `/docs/` subdirectory or rename
it. The latter would make sense if we split, say, `/product/` from
`/icons/` and/or `/brand`.
[Primer](https://primer.style/octicons/design-guidelines/) does this
well.

apps/design-system/app/(app)/docs/[[...slug]]/page.tsx

@@ -1,5 +1,5 @@
 import { Mdx } from '@/components/mdx-components'
-import { DocsPager } from '@/components/pager'
+import { DocsPager, getBreadcrumbSegments } from '@/components/pager'
 import { SourcePanel } from '@/components/source-panel'
 import { DashboardTableOfContents } from '@/components/toc'
 import { siteConfig } from '@/config/site'
@@ -12,6 +12,7 @@ import '@/styles/mdx.css'
 import { allDocs } from 'contentlayer/generated'
 import { ChevronRight } from 'lucide-react'
 import type { Metadata } from 'next'
+import Link from 'next/link'
 import { notFound } from 'next/navigation'
 import Balancer from 'react-wrap-balancer'
 import { ScrollArea, Separator } from 'ui'
@@ -83,15 +84,39 @@ export default async function DocPage(props: DocPageProps) {
   }
 
   const toc = await getTableOfContents(doc.body.raw)
+  const breadcrumbSegments = getBreadcrumbSegments(doc)
 
   return (
     <div className="relative xl:grid xl:grid-cols-[1fr_160px] gap-4 px-6 py-6 lg:py-8">
       <div className="mx-auto w-full min-w-0 max-w-4xl flex-1">
-        <div className="mb-4 flex items-center space-x-1 text-sm text-foreground-muted">
-          <div className="overflow-hidden text-ellipsis whitespace-nowrap">Docs</div>
-          <ChevronRight className="h-4 w-4 text-foreground-muted" />
-          <div className="text-foreground-lighter">{doc.title}</div>
-        </div>
+        <nav
+          className="mb-4 flex items-center space-x-1 text-sm text-foreground-lighter"
+          aria-label="Breadcrumb"
+        >
+          {breadcrumbSegments.map((segment, index) => (
+            <span key={segment.title} className="flex items-center gap-1">
+              {index > 0 && <ChevronRight className="h-4 w-4 shrink-0 text-foreground-muted" />}
+              {segment.href ? (
+                <Link
+                  href={segment.href}
+                  className="overflow-hidden text-ellipsis whitespace-nowrap hover:text-foreground transition-colors"
+                >
+                  {segment.title}
+                </Link>
+              ) : (
+                <span className="overflow-hidden text-ellipsis whitespace-nowrap text-foreground-lighter">
+                  {segment.title}
+                </span>
+              )}
+            </span>
+          ))}
+          {breadcrumbSegments.length > 0 && (
+            <ChevronRight className="h-4 w-4 shrink-0 text-foreground-muted/50" />
+          )}
+          <span className="overflow-hidden text-ellipsis whitespace-nowrap text-foreground-muted">
+            {doc.title}
+          </span>
+        </nav>
         <div className="space-y-2 mb-5">
           <h1 className="scroll-m-20 text-4xl tracking-tight">{doc.title}</h1>
           {doc.description && (

apps/design-system/components/pager.tsx

@@ -71,3 +71,27 @@ export function flatten(links: NavItemWithChildren[]): NavItem[] {
     }, [])
     .filter((link) => !link?.disabled)
 }
+
+export interface BreadcrumbSegment {
+  title: string
+  href?: string
+}
+
+export function getBreadcrumbSegments(doc: Doc): BreadcrumbSegment[] {
+  const segments: BreadcrumbSegment[] = [{ title: 'Docs', href: '/docs' }]
+
+  for (const section of docsConfig.sidebarNav) {
+    const flatItems = flatten(section.items ?? [])
+    const inSection = flatItems.some((item) => item.href === doc.slug)
+    if (!inSection || !section.title) continue
+
+    const sectionLanding = flatItems.find((item) => item.priority) ?? flatItems[0]
+    const isOnLanding = sectionLanding?.href === doc.slug
+    if (sectionLanding?.href && !isOnLanding) {
+      segments.push({ title: section.title, href: sectionLanding.href })
+    }
+    break
+  }
+
+  return segments
+}