Update Colab badge URLs to support notebooks on gh-pages

Background

As part of the QuantEcon Actions migration (see QuantEcon/actions#25 and PLAN.md), we're simplifying our notebook deployment architecture.

Current design:

• Lecture repos (e.g., lecture-python.myst) build notebooks via sphinx-tojupyter
• Notebooks are synced to separate .notebooks repos (e.g., lecture-python.notebooks)
• Colab badges link to: https://colab.research.google.com/github/QuantEcon/{repo}.notebooks/blob/main/{path}

New design:

• Notebooks are deployed to gh-pages alongside HTML content in _notebooks/ directory
• Colab badges link to: https://colab.research.google.com/notebook?url=https://{domain}/_notebooks/{path}
• Eliminates 4 separate .notebooks repos and sync workflows

Requested Changes

Update Colab badge/link generation to support notebooks hosted on gh-pages:

Current URL format:

https://colab.research.google.com/github/QuantEcon/lecture-python.notebooks/blob/main/example.ipynb

New URL format:

https://colab.research.google.com/notebook?url=https://python.quantecon.org/_notebooks/example.ipynb

Implementation Notes

• Google Colab supports opening notebooks from any public URL via the url parameter
• The {domain} should be configurable per lecture series:
  • lecture-python.myst → python.quantecon.org
  • lecture-python-programming.myst → python-programming.quantecon.org
  • lecture-python-intro → (Netlify domain)
  • lecture-python-advanced.myst → python-advanced.quantecon.org
• Notebooks are already copied to _build/html/_notebooks/ via the build-lectures action with html-copy-notebooks: 'true'

Benefits

• ✅ Reduces repo count from 8 to 4 (eliminates .notebooks repos)
• ✅ Eliminates sync step in publish workflows
• ✅ Single source of truth for notebooks
• ✅ Simpler maintenance

Related

• QuantEcon/actions PLAN.md — Migration strategy
• QuantEcon/actions PR #25 — ML libs removal
• Primary user access: (1) Download links on lecture pages, (2) Colab badges on lecture pages

Timeline

Needed for upcoming lecture repository migrations (Q1 2026).

cc @mmcky

Read PLAN.md and review the build-lectures configuration that copies notebooks into _build/html/_notebooks/. Trace the existing Colab badge generation and its current GitHub URL format. Done means configurable lecture domains produce the new notebook?url links for notebooks hosted under _notebooks/.