docs: fern migration

fern/README.md

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+# NeMo RL Fern Documentation
+
+This folder contains the Fern Docs configuration for NeMo RL.
+
+## Installation
+
+```bash
+npm install -g fern-api
+# Or: npx fern-api --version
+```
+
+## Local Preview
+
+```bash
+cd fern/
+fern docs dev
+# Or from project root: fern docs dev --project ./fern
+```
+
+Docs available at `http://localhost:3000`.
+
+## Folder Structure
+
+```
+fern/
+├── docs.yml              # Global config (title, colors, versions)
+├── fern.config.json      # Fern CLI config
+├── versions/
+│   └── v0.5.0.yml       # Navigation for v0.5.0
+├── v0.5.0/
+│   └── pages/            # MDX content for v0.5.0
+├── scripts/              # Migration and conversion scripts
+└── assets/               # Favicon, images
+```
+
+## Migration Workflow
+
+To migrate or update docs from `docs/` to Fern:
+
+**Assets:** The docs reference images (e.g. `../assets/*.png`). These must exist in `docs/assets/` and will be copied to `fern/assets/` by the copy script. If `docs/assets/` is missing or images are not committed, create the directory and add the image files, or the Fern build will report missing path errors. Image paths in MDX are normalized to `/assets/` (relative to the Fern site root).
+
+```bash
+# 1. Copy docs to fern (run from repo root)
+python3 fern/scripts/copy_docs_to_fern.py v0.5.0
+
+# 2. Convert RL-specific syntax first (octicon, py:class, py:meth)
+python3 fern/scripts/convert_rl_specific.py fern/v0.5.0/pages
+
+# 3. Convert MyST to Fern MDX
+python3 fern/scripts/convert_myst_to_fern.py fern/v0.5.0/pages
+
+# 4. Add frontmatter
+python3 fern/scripts/add_frontmatter.py fern/v0.5.0/pages
+
+# 5. Update internal links
+python3 fern/scripts/update_links.py fern/v0.5.0/pages
+
+# 6. Remove duplicate H1s (when title matches frontmatter)
+python3 fern/scripts/remove_duplicate_h1.py fern/v0.5.0/pages
+
+# 7. Validate
+./fern/scripts/check_unconverted.sh fern/v0.5.0/pages
+uv run python fern/scripts/find_tag_mismatches.py fern/v0.5.0/pages
+```
+
+## Bumping the Version
+
+When releasing a new version (e.g., v0.6.0):
+
+1. Copy the previous version's content:
+   ```bash
+   cp -r fern/v0.5.0 fern/v0.6.0
+   ```
+
+2. Create the navigation file:
+   ```bash
+   cp fern/versions/v0.5.0.yml fern/versions/v0.6.0.yml
+   ```
+
+3. In `versions/v0.6.0.yml`: replace `../v0.5.0/pages/` → `../v0.6.0/pages/`
+
+4. In `docs.yml`: add the new version to the `versions:` list
+
+5. Make content changes in `fern/v0.6.0/pages/`
+
+## MDX Components
+
+```mdx
+<Note>Informational note</Note>
+<Tip>Helpful tip</Tip>
+<Warning>Warning message</Warning>
+<Info>Info callout</Info>
+
+<Cards>
+  <Card title="Title" href="/path">Description</Card>
+</Cards>
+
+<Tabs>
+  <Tab title="Python">```python\ncode\n```</Tab>
+</Tabs>
+```
+
+## API Reference
+
+API docs are built by Sphinx (autodoc2) and hosted at docs.nvidia.com. The "API Reference" link in the navbar points to `https://docs.nvidia.com/nemo/rl/latest/apidocs/`.
+
+## Deploying
+
+```bash
+fern generate --docs
+fern docs deploy
+```
+
+## Useful Links
+
+- [Fern Docs](https://buildwithfern.com/learn/docs)
+- [MDX Components](https://buildwithfern.com/learn/docs/components)
+- [Versioning Guide](https://buildwithfern.com/learn/docs/configuration/versions)

fern/assets/README.md

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+# Fern Assets
+
+Add `favicon.png` here for the docs site logo and favicon.
+See NeMo Curator or DataDesigner fern/assets for reference.