fix(docs): stabilize CI documentation builds

[pablormier]

Summary

• keep the Gurobi PCST guide indexed without executing it on unlicensed CI runners
• preserve local RC and migration pages when synchronizing published releases
• add a focused regression test for release-index generation

Validation

• poetry run pytest -q tests/test_sync_releases.py
• poetry run ruff check docs/conf.py scripts/sync_releases.py tests/test_sync_releases.py
• poetry run ruff format --check docs/conf.py scripts/sync_releases.py tests/test_sync_releases.py
• poetry run sphinx-build -W --keep-going -b html docs docs/_build/html