Add hidden sandbox test page for component preview testing

fern/products/docs/docs.yml

@@ -461,6 +461,13 @@ navigation:
       - page: Stainless vs. Fern
         path: ./pages/resources/stainless-comparison.mdx
         slug: stainless-comparison
+  - section: Sandbox
+    collapsed: true
+    hidden: true
+    contents:
+      - page: Test page sandbox
+        path: ./pages/sandbox/test-page.mdx
+        slug: test-page
   - api: Public API
     paginated: false
     api-name: mcp-tools

fern/products/docs/pages/sandbox/test-page.mdx

@@ -0,0 +1,140 @@
+---
+title: Test page sandbox
+subtitle: A non-public sandbox page used for exercising components and the preview pipeline.
+description: A non-public sandbox page used for exercising Fern components and the preview pipeline.
+hide-toc: false
+hide-feedback: true
+hide-nav-links: true
+---
+
+This page is an internal sandbox. It exists to exercise a representative slice of Fern Docs components in a single place — useful for verifying preview deployments, theming changes, and component rendering without editing a real customer-facing page.
+
+<Note>
+  This page is hidden from the sidebar. It is reachable by direct URL only and is not linked from public navigation.
+</Note>
+
+## Callouts
+
+<Info>An informational callout for drawing attention to context.</Info>
+<Warning>A warning callout for non-blocking risks the reader should consider.</Warning>
+<Tip>A tip callout for non-essential suggestions that improve outcomes.</Tip>
+<Success>A success callout for confirming a positive outcome.</Success>
+<Error>An error callout for surfacing failure conditions.</Error>
+
+## Code blocks
+
+A multi-language code block group demonstrates syntax highlighting and the language switcher:
+
+<CodeBlocks>
+  ```bash title="curl"
+  curl -X POST https://api.example.com/v1/things \
+    -H "Authorization: Bearer $TOKEN" \
+    -H "Content-Type: application/json" \
+    -d '{"name": "widget", "quantity": 3}'
+  ```
+
+  ```typescript title="TypeScript"
+  import { ExampleClient } from "@example/sdk";
+
+  const client = new ExampleClient({ token: process.env.TOKEN });
+
+  const thing = await client.things.create({
+    name: "widget",
+    quantity: 3,
+  });
+  ```
+
+  ```python title="Python"
+  from example import ExampleClient
+
+  client = ExampleClient(token=os.environ["TOKEN"])
+
+  thing = client.things.create(name="widget", quantity=3)
+  ```
+</CodeBlocks>
+
+## Tabs
+
+<Tabs>
+  <Tab title="Overview">
+    Tabs let you present the same concept from multiple angles — for example, audience (beginner vs. advanced), runtime (Node vs. Browser), or workflow (CLI vs. UI).
+  </Tab>
+  <Tab title="When to use">
+    Use tabs when content is logically parallel and a reader is likely to want only one variant at a time. Avoid tabs for sequential steps.
+  </Tab>
+  <Tab title="When not to use">
+    Avoid tabs for content that should be discoverable via search or skimming. Hidden tabs are easy to miss.
+  </Tab>
+</Tabs>
+
+## Accordion
+
+<AccordionGroup>
+  <Accordion title="Why does this page exist?">
+    To validate component rendering, preview deployments, and CI without touching customer-facing pages.
+  </Accordion>
+  <Accordion title="Is it indexed?">
+    No. It is marked `hidden: true` in navigation and is not linked from the sidebar.
+  </Accordion>
+  <Accordion title="Can it be deleted?">
+    Yes. This page has no dependencies and can be removed once the change it was created for has shipped.
+  </Accordion>
+</AccordionGroup>
+
+## Cards
+
+<CardGroup cols={2}>
+  <Card title="Configuration overview" icon="fa-duotone fa-gear" href="/learn/docs/configuration/overview">
+    Site- and page-level configuration for a Fern Docs project.
+  </Card>
+  <Card title="Components overview" icon="fa-duotone fa-book" href="/learn/docs/writing-content/components/overview">
+    The full catalog of built-in components.
+  </Card>
+  <Card title="Markdown basics" icon="fa-duotone fa-pen" href="/learn/docs/writing-content/markdown-basics">
+    Reference for the Markdown and MDX features Fern supports.
+  </Card>
+  <Card title="Project structure" icon="fa-duotone fa-folder-tree" href="/learn/docs/getting-started/project-structure">
+    How files and folders are laid out in a Fern Docs project.
+  </Card>
+</CardGroup>
+
+## Tables
+
+| Component       | Purpose                                     | Common props                  |
+| --------------- | ------------------------------------------- | ----------------------------- |
+| `<Callout>`     | Highlight important context                 | `intent`, `icon`, `title`     |
+| `<CodeBlocks>`  | Show the same snippet in multiple languages | `title` (per child)           |
+| `<Tabs>`        | Parallel variants of one concept            | `title` (per child)           |
+| `<Accordion>`   | Progressive disclosure                      | `title`                       |
+| `<Card>`        | Linked navigation tile                      | `title`, `icon`, `href`       |
+
+## Parameter fields
+
+<ParamField path="example_id" type="string" required={true}>
+  A unique identifier used to look up the example.
+</ParamField>
+
+<ParamField path="limit" type="integer" required={false} default="20">
+  The maximum number of items to return in a single response.
+</ParamField>
+
+<ParamField path="include_archived" type="boolean" required={false} default="false">
+  Whether to include archived items in the response.
+</ParamField>
+
+## Steps
+
+<Steps>
+  ### Install the SDK
+  Add the SDK to your project with your package manager of choice.
+
+  ### Configure credentials
+  Set the `TOKEN` environment variable to a valid API token.
+
+  ### Make a request
+  Call the API and inspect the response. If you receive a 401, double-check the token value and scope.
+</Steps>
+
+<llms-only>
+  This is a non-production sandbox page. Agents inspecting the docs corpus can safely ignore it — it does not document any customer-facing behavior.
+</llms-only>