docs: fern docs migration

fern/README.md

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+# Fern Documentation Cheat Sheet
+
+This folder contains the Fern Docs configuration for NeMo Data Designer.
+
+## 📦 Installation
+
+```bash
+# Install Fern CLI globally
+npm install -g fern-api
+
+# Or use npx (no install needed)
+npx fern-api --version
+```
+
+## 🔍 Local Preview
+
+```bash
+# From the fern/ directory
+cd fern/
+fern docs dev
+
+# Or from project root
+fern docs dev --project ./fern
+```
+
+The docs will be available at `http://localhost:3000`.
+
+## 📁 Folder Structure
+
+```
+fern/
+├── docs.yml              # Global config (title, colors, versions)
+├── fern.config.json      # Fern CLI config (org name)
+├── versions/
+│   ├── v0.3.3.yml        # Navigation for v0.3.3
+│   └── v0.4.0.yml        # Navigation for v0.4.0
+├── v0.3.3/
+│   └── pages/            # MDX content for v0.3.3
+├── v0.4.0/
+│   └── pages/            # MDX content for v0.4.0
+└── assets/               # Shared images, favicons
+```
+
+## 🔄 Bumping the Version
+
+When releasing a new version (e.g., v0.5.0):
+
+### 1. Copy the previous version's content
+```bash
+cp -r fern/v0.4.0 fern/v0.5.0
+```
+
+### 2. Create the navigation file
+```bash
+cp fern/versions/v0.4.0.yml fern/versions/v0.5.0.yml
+```
+
+### 3. Update paths in `versions/v0.5.0.yml`
+Change all `../v0.4.0/pages/` → `../v0.5.0/pages/`
+
+### 4. Add the new version to `docs.yml`
+```yaml
+versions:
+  - display-name: v0.5.0
+    path: versions/v0.5.0.yml
+    slug: v0.5.0
+  - display-name: v0.4.0
+    path: versions/v0.4.0.yml
+    slug: v0.4.0
+  # ... older versions
+```
+
+### 5. Make your content changes
+Edit files in `fern/v0.5.0/pages/`
+
+## ✏️ Editing Content
+
+### Adding a new page
+
+1. Create the MDX file in the appropriate version folder:
+   ```bash
+   touch fern/v0.3.3/pages/concepts/new-feature.mdx
+   ```
+
+2. Add frontmatter:
+   ```mdx
+   ---
+   title: New Feature
+   description: Description for SEO.
+   ---
+
+   Content starts here...
+   ```
+
+3. Add to navigation in `versions/v0.3.3.yml`:
+   ```yaml
+   - page: New Feature
+     path: ../v0.3.3/pages/concepts/new-feature.mdx
+   ```
+
+### MDX Components
+
+```mdx
+# Callouts
+<Note>Informational note</Note>
+<Tip>Helpful tip</Tip>
+<Warning>Warning message</Warning>
+<Info>Info callout</Info>
+
+# Tabs
+<Tabs>
+  <Tab title="Python">
+    ```python
+    print("hello")
+    ```
+  </Tab>
+  <Tab title="JavaScript">
+    ```javascript
+    console.log("hello")
+    ```
+  </Tab>
+</Tabs>
+
+# Cards
+<Cards>
+  <Card title="Title" href="/path">
+    Description
+  </Card>
+</Cards>
+```
+
+## 🚀 Deploying
+
+```bash
+# Generate static docs (for CI/CD)
+fern generate --docs
+
+# Deploy to Fern hosting
+fern docs deploy
+```
+
+## 🔗 Useful Links
+
+- [Fern Docs](https://buildwithfern.com/learn/docs)
+- [MDX Components Reference](https://buildwithfern.com/learn/docs/components)
+- [Versioning Guide](https://buildwithfern.com/learn/docs/configuration/versions)
+- [Navigation Configuration](https://buildwithfern.com/learn/docs/configuration/navigation)
+
+## ⚠️ Common Issues
+
+### "EISDIR: illegal operation on a directory"
+- Check that all `path:` values point to `.mdx` files, not directories
+
+### Page not showing
+- Verify the page is listed in the version's navigation file
+- Check the path is correct (relative to the versions/ folder)
+
+### Version selector not appearing
+- Ensure `versions:` is defined in `docs.yml`
+- Each version needs a valid `.yml` file in `versions/`