chore: deprecate MkDocs docs

[andreatgretel]

📋 Summary

Deprecate MkDocs as an active docs system now that Fern is the canonical Data Designer docs surface. This freezes the legacy GitHub Pages archive for 0.5.7 and older while removing the source, CI, dependencies, and contributor guidance that kept generating new MkDocs builds.

🔗 Related Issue

N/A

🔄 Changes

✨ Added

• Added docs/README.md to make the remaining docs/ directory explicitly support-only for notebook sources, Colab notebooks, docs scripts, and downloadable recipe scripts.
• Documented the frozen legacy gh-pages archive in VERSIONING.md.

🔧 Changed

• Converted docs-preview.yml to Fern-only preview/check behavior and removed the Cloudflare MkDocs preview deploy.
• Removed the deleted build-docs.yml fallback from publish-fern-devnotes.yml.
• Dropped MkDocs/mike/mkdocstrings dependencies from pyproject.toml and refreshed uv.lock.
• Updated README, Fern maintenance docs, agent docs-search guidance, tutorial notebook sources, and generated Colab notebooks to point current readers at docs.nvidia.com/nemo/datadesigner.

🗑️ Removed

• Removed mkdocs.yml, .github/workflows/build-docs.yml, .github/workflows/publish-devnotes.yml, and the MkDocs-only devnote nav patch helper.
• Removed legacy MkDocs prose/assets under docs/code_reference/, docs/concepts/, docs/devnotes/, docs/images/, docs/plugins/, docs/recipes/, docs/css/, docs/js/, docs/overrides/, and docs/index.md.
• Removed the stale /site MkDocs build-output ignore entry.

🔍 Attention Areas

⚠️ Reviewers: Please pay special attention to the following:

• docs/ — this PR intentionally keeps Fern support inputs there instead of moving fern/ under docs/; moving Fern can happen separately.
• docs-preview.yml — now only posts Fern previews and no longer publishes a MkDocs/Cloudflare preview.
• fern/docs.yml — existing redirects remain the active bridge to current Fern docs and the frozen MkDocs archives for 0.5.7 and older.

🧪 Testing

• .venv/bin/ruff check --fix .
• .venv/bin/ruff format .
• make check-fern-docs-locally (fern check: 0 errors, 2 existing warnings)
• make generate-colab-notebooks
• git diff --check HEAD
• make test passes (not run; docs/CI cleanup only)
• Unit tests added/updated (N/A - no runtime logic changes)
• E2E tests added/updated (N/A - no runtime logic changes)

✅ Checklist

• Follows commit message conventions
• Commits are signed off (DCO)
• Architecture docs updated (N/A - no architecture changes)

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Description updated with AI

[github-actions]

Fern preview: https://nvidia-preview-pr-687.docs.buildwithfern.com/nemo/datadesigner

Fern previews include the docs-website version archive with PR changes synced into latest. Notebook tutorials are rendered without execution outputs in previews.