Summary

Deprecates MkDocs as an active docs system on main now that Fern is the canonical Data Designer docs surface.

This PR does not modify the published gh-pages archive. The archive freeze and GitHub Pages redirect behavior live in #688.

Changes

• Remove MkDocs source pages/assets, mkdocs.yml, and MkDocs-only docs workflows from main.
• Convert docs preview and devnotes publishing follow-up to Fern-only behavior.
• Drop MkDocs/mike/mkdocstrings docs dependencies and refresh uv.lock.
• Keep docs/ for Fern support inputs only: notebook sources, generated Colab notebooks, docs scripts, and downloadable recipe assets.
• Update current docs links to https://docs.nvidia.com/nemo/datadesigner.
• Keep the Fern redirect mapping for legacy MkDocs/devnotes URLs in fern/docs.yml.

Archive Boundary

• chore: deprecate MkDocs docs #687 targets main and stops new MkDocs builds from this branch.
• docs: freeze gh-pages archive #688 targets gh-pages and freezes the GitHub Pages archive so only 0.5.7 and older remain available there.
• docs: freeze gh-pages archive #688 also implements the latest, 0.6.0, 0.5.9, and 0.5.8 GitHub Pages redirects to Fern.

Attention Areas

• docs/ intentionally remains as a support directory for Fern-linked assets and notebooks.
• .github/workflows/docs-preview.yml now posts Fern previews only.
• fern/docs.yml remains the active Fern-side bridge for legacy MkDocs/devnotes paths.

Testing

• .venv/bin/ruff check --fix packages/data-designer/src/data_designer/interface/data_designer.py docs/notebook_source/*.py
• .venv/bin/ruff format packages/data-designer/src/data_designer/interface/data_designer.py docs/notebook_source/*.py
• make generate-colab-notebooks
• make check-fern-docs (fern check: 0 errors, 1 warning)
• git diff --check origin/main..HEAD
• make test passes (not run locally; docs/CI cleanup only)
• Unit tests added/updated (N/A - no runtime logic changes)
• E2E tests added/updated (N/A - no runtime logic changes)