Fix linkcheck false positives for FRED (timeout) and update redirect URLs

[Copilot]

The automated link checker was reporting 2 broken links (FRED timeout) and 2 redirects (mypy, JAX) on every scheduled run.

Changes

• Redirects already resolved: python_advanced_features.md already uses the final destination URLs (https://mypy.readthedocs.io/en/stable/, https://docs.jax.dev/en/latest/) — no content changes needed.

• FRED false positive fix (linkcheck.yml): Added a pre-processing step before the link checker runs that neutralises fred.stlouisfed.org hrefs → href="#". FRED actively blocks automated crawlers, so these timeouts are false positives. The skip list is driven by a SKIP_DOMAINS env var for easy extension:

- name: Pre-process HTML to exclude known bot-blocking domains
  env:
    SKIP_DOMAINS: "fred.stlouisfed.org"
  run: |
    python3 - <<'PYEOF'
    import os, re
    from pathlib import Path
    skip_domains = os.environ.get('SKIP_DOMAINS', '').split()
    for html_file in Path('_site').rglob('*.html'):
        ...  # replace matching hrefs with href="#"
    PYEOF

This mirrors the existing linkcheck_ignore: ['https://fred.stlouisfed.org/.*'] already present in lectures/_config.yml for Sphinx's built-in checker.