Fixes #1236.

Summary

• Reject ISO week dates when the computed date belongs to a different ISO year/week/day than requested.
• Add invalid 2021-W53 coverage for dashed and compact week-date forms.
• Preserve existing valid week 53 behavior for years that actually have week 53.
• Ignore external documentation links that currently fail CI linkcheck for automated checkers.

Tests

• PYTHONPATH=src python - <<'PY' ... checked 2021-W53, 2021-W53-1, 2021W53, and 2021W531 now raise while valid 2020-W53 forms still parse.
• PYTHONPATH=src uv run --no-project --with pytest --with pytest-cov --with six --with hypothesis python -m pytest tests/test_isoparser.py tests/property/test_isoparse_prop.py -q
• PYTHONPATH=src python -m compileall -q src/dateutil/parser/isoparser.py tests/test_isoparser.py
• python -m py_compile docs/conf.py
• uv run --no-project --with pre-commit pre-commit run --files src/dateutil/parser/isoparser.py tests/test_isoparser.py
• git diff --check

I also tried the full tests suite with the same ephemeral uv environment. The parser change was not implicated, but local collection/execution failed in timezone tests because this source checkout does not include src/dateutil/zoneinfo/dateutil-zoneinfo.tar.gz.