Add basic error boundary documentation (#5605)

Add Error Boundaries documentation for React and Solid

Co-authored-by: Cursor Agent <cursoragent@cursor.com>

Author: tannerlinsley

docs/start/config.json

@@ -97,6 +97,10 @@
               "label": "Middleware",
               "to": "framework/react/guide/middleware"
             },
+            {
+              "label": "Error Boundaries",
+              "to": "framework/react/guide/error-boundaries"
+            },
             {
               "label": "Server Routes",
               "to": "framework/react/guide/server-routes"
@@ -190,6 +194,10 @@
               "label": "Middleware",
               "to": "framework/solid/guide/middleware"
             },
+            {
+              "label": "Error Boundaries",
+              "to": "framework/solid/guide/error-boundaries"
+            },
             {
               "label": "Server Routes",
               "to": "framework/solid/guide/server-routes"

docs/start/framework/react/guide/error-boundaries.md

@@ -0,0 +1,52 @@
+---
+id: error-boundaries
+$title: Error Boundaries
+---
+
+## Error Boundaries (React Start)
+
+TanStack Start uses TanStack Router's route-level error boundaries.
+
+- Set a default for all routes via the router
+- Override per-route with `errorComponent`
+
+### Configure a default
+
+```tsx
+// src/router.tsx
+import { createRouter, ErrorComponent } from '@tanstack/react-router'
+import { routeTree } from './routeTree.gen'
+
+export function getRouter() {
+  const router = createRouter({
+    routeTree,
+    // Shown when an error bubbles to the router
+    defaultErrorComponent: ({ error, reset }) => (
+      <ErrorComponent error={error} />
+    ),
+  })
+  return router
+}
+```
+
+### Per-route override
+
+```tsx
+// src/routes/posts.$postId.tsx
+import { createFileRoute, ErrorComponent } from '@tanstack/react-router'
+import type { ErrorComponentProps } from '@tanstack/react-router'
+
+function PostError({ error, reset }: ErrorComponentProps) {
+  return <ErrorComponent error={error} />
+}
+
+export const Route = createFileRoute('/posts/$postId')({
+  component: PostComponent,
+  errorComponent: PostError,
+})
+```
+
+Notes:
+- `ErrorComponent` is a simple built-in UI you can replace.
+- Call `reset()` to retry rendering the route after fixing state.
+- Use `beforeLoad`/`loader` to throw errors that will be caught.

docs/start/framework/solid/guide/error-boundaries.md

@@ -0,0 +1,51 @@
+---
+id: error-boundaries
+$title: Error Boundaries
+---
+
+## Error Boundaries (Solid Start)
+
+TanStack Start (Solid) uses TanStack Router's route-level error boundaries.
+
+- Set a default for all routes via the router
+- Override per-route with `errorComponent`
+
+### Configure a default
+
+```tsx
+// src/router.tsx
+import { createRouter, ErrorComponent } from '@tanstack/solid-router'
+import { routeTree } from './routeTree.gen'
+
+export function getRouter() {
+  const router = createRouter({
+    routeTree,
+    defaultErrorComponent: ({ error, reset }) => (
+      <ErrorComponent error={error} />
+    ),
+  })
+  return router
+}
+```
+
+### Per-route override
+
+```tsx
+// src/routes/posts.$postId.tsx
+import { createFileRoute, ErrorComponent } from '@tanstack/solid-router'
+import type { ErrorComponentProps } from '@tanstack/solid-router'
+
+function PostError(props: ErrorComponentProps) {
+  return <ErrorComponent error={props.error} />
+}
+
+export const Route = createFileRoute('/posts/$postId')({
+  component: PostComponent,
+  errorComponent: PostError,
+})
+```
+
+Notes:
+- `ErrorComponent` is a simple built-in UI you can replace.
+- Call `reset()` to retry rendering the route after fixing state.
+- Use `beforeLoad`/`loader` to throw errors that will be caught.