Clarify restricted dataset access setup

[MaxGhenis]

Summary

• explain that access to restricted UK datasets is granted separately from creating a Hugging Face token
• document the current request path via contact@policyengine.org, including the need to provide a Hugging Face username and proof of UKDS-backed access
• add a changelog entry

Testing

• cd docs/book && jupyter book clean --all -y
  Deleting all the following paths:

  • /Users/maxghenis/worktrees/policyengine-uk-fix-1115/_build/html
  • /Users/maxghenis/worktrees/policyengine-uk-fix-1115/_build/site
  • /Users/maxghenis/worktrees/policyengine-uk-fix-1115/_build/templates

🗑 Deleting: /Users/maxghenis/worktrees/policyengine-uk-fix-1115/_build/html
🗑 Deleting: /Users/maxghenis/worktrees/policyengine-uk-fix-1115/_build/site
🗑 Deleting: /Users/maxghenis/worktrees/policyengine-uk-fix-1115/_build/templates
cd docs/book && jupyter book build --html
🌎 Building Jupyter Book (via myst) site
🔍 Querying template metadata from https://api.mystmd.org/templates/site/myst/book-theme
🐕 Fetching template from https://github.com/myst-templates/book-theme/archive/refs/heads/main.zip
💾 Saved template to path /Users/maxghenis/worktrees/policyengine-uk-fix-1115/_build/templates/site/myst/book-theme
Building the base site.
To set a baseurl (e.g. GitHub pages) use "BASE_URL" environment variable.
⤵️ Installing web libraries (can take up to 60 s)
📦 Installed web libraries in 874 ms
📖 Built index.md in 56 ms.
📖 Built assumptions/water-bills.md in 52 ms.
📖 Built policy/model-baseline.md in 55 ms.
📖 Built usage/scenarios.md in 63 ms.
📖 Built policy/uc-rebalancing.md in 58 ms.
📖 Built usage/getting-started.md in 68 ms.
📖 Built validation/hbai.md in 68 ms.
📖 Built usage/dynamics.md in 73 ms.
📖 Built assumptions/growthfactors.md in 80 ms.
📖 Built usage/simulations.md in 85 ms.
📚 Built 10 pages for project in 123 ms.

✨✨✨  Starting Book Theme  ✨✨✨

🔌 Server started on port 3003! 🥳 🎉

👉  http://localhost:3003  👈

💌 GET /dynamics 200 - - 29.751 ms
GET /getting-started 200 - - 38.708 ms
GET / 200 - - 35.107 ms
GET /simulations 200 - - 42.343 ms
GET /scenarios 200 - - 44.899 ms
💌 GET /water-bills 200 - - 16.182 ms
GET /hbai 200 - - 20.990 ms
GET /uc-rebalancing 200 - - 30.565 ms
GET /model-baseline 200 - - 27.568 ms
GET /growthfactors 200 - - 13.085 ms
💌 GET /index.json 200 - - 5.407 ms
GET /getting-started.json 200 - - 4.705 ms
GET /scenarios.json 200 - - 4.661 ms
GET /dynamics.json 200 - - 4.949 ms
GET /simulations.json 200 - - 5.597 ms
💌 GET /water-bills.json 200 - - 3.728 ms
GET /growthfactors.json 200 - - 5.874 ms
💌 GET /hbai.json 200 - - 5.866 ms
GET /uc-rebalancing.json 200 - - 5.882 ms
GET /model-baseline.json 200 - - 6.316 ms
💌 GET /robots.txt 200 - - 0.510 ms
💌 GET /myst-theme.css 200 - - 8.720 ms
💌 GET /favicon.ico 200 - - 75.656 ms
python docs/book/add_plotly_to_book.py docs/book/_build

Closes #1115.

[policyengine]

This PR effectively addresses issue #1115 by clarifying the restricted dataset access process. The changes are clear and well-structured:

What was changed:

• Added a new step explicitly stating that users need to request access from PolicyEngine before generating a token
• Clarified what information to include in the request (HuggingFace username + proof of UKDS-backed access)
• Added a helpful note explaining that PolicyEngine grants access to the repository but doesn't issue tokens directly
• Included proper changelog entry

Quality checks:

• Documentation is clear and follows a logical step-by-step flow
• The contact email link is properly formatted
• Changelog entry follows the correct naming pattern (hf-token-request-docs.fixed.md)
• The warning callout appropriately flags this as important setup information
• Testing shows the Jupyter book builds successfully

No bugs, security issues, or logic errors found. The documentation accurately reflects the access control model where PolicyEngine grants HuggingFace repository access, then users create their own tokens.