Fix "Update website" CI: upgrade Python 3.8 → 3.12 and Sphinx 4 → 7

[Copilot]

The "Update website" workflow fails because dependencies (mistune 3.3.1, then Sphinx 4.0.2 itself) use type annotation syntax incompatible with Python 3.8/3.12 respectively. Python 3.8 is EOL.

Changes

• .github/workflows/update-website.yml: Bump Python from 3.8 to 3.12
• notes/requirements.txt: Upgrade sphinx==4.0.2 → sphinx>=7.4,<8 (Sphinx 4 cannot import on Python 3.12: from types import Union no longer exists)
• notes/source/conf.py: Update extlinks to Sphinx 7 format (requires %s placeholder in URL pattern and caption)

# Before (Sphinx 4 format)
extlinks = {'pdf_link': ('pdf/code-packaging-rev{}.pdf'.format(git_hash), 'PDF version')}

# After (Sphinx 7 format)
extlinks = {'pdf_link': ('pdf/code-packaging-rev{}.pdf%s'.format(git_hash), 'PDF version%s')}