docs(devnotes): add Nemotron-Personas dev note

[3mei]

📋 Summary

Adds the Inside Nemotron-Personas dev note covering how the multi-locale Nemotron-Personas HF collection is built (4-stage compound-AI pipeline) and how it's used as a seeding primitive across Nemotron training (long-context, tool-use, formal logic, safety refusals, instruction-following). Ships alongside a runnable Tutorial 7 demonstrating reproduction + customization, plus a Colab variant

🔗 Related Issue

N/A

🔄 Changes

✨ Added

• docs/devnotes/posts/nemotron-personas.md — new dev note
• docs/devnotes/posts/assets/nemotron-personas/ — four images: three pipeline-stage diagrams from the partner repo plus a black-background Nemotron-Personas world-map hero
• docs/notebook_source/7-nemotron-personas.py — jupytext source for the Reproducing & Customizing Nemotron-Personas tutorial;
• docs/colab_notebooks/7-nemotron-personas.ipynb — committed Colab variant; i

🔧 Changed

• docs/scripts/generate_colab_notebooks.py — adds an ADDITIONAL_SETUP_CELLS map paralleling ADDITIONAL_DEPENDENCIES; injects NGC CLI install + NGC_API_KEY cells. Future devnote-paired tutorials needing extra Colab bootstrap can register one-line entries in the same map.
• mkdocs.yml — adds Reproducing & Customizing Nemotron-Personas under the Tutorials nav

🧪 Testing

• make test passes
• Notebook runs end-to-end via jupytext --to ipynb --execute
• make generate-colab-notebooks regenerates the Colab .ipynb cleanly with the NGC setup cells in the expected position
• Unit tests added/updated (N/A — this PR is docs + tutorial assets; no engine code changed)
• E2E tests added/updated (N/A — Tutorial 7 is opt-in via make convert-execute-notebooks and gated on NVIDIA_API_KEY + on-disk NGC dataset, matching how Tutorials 5/6 are gated on OPENROUTER_API_KEY)

✅ Checklist

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

[github-actions]

Docs preview: https://06744256.dd-docs-preview.pages.dev

Notebook tutorials are placeholder-only in previews.