Document rule zip setup for external contributors (no gim-home access)

[timenick]

Problem

The runtime check rule zips required by winml analyze are hosted in gim-home/ModelKitArtifacts, which is not accessible to external contributors. The current setup docs in runtime_check_rules/README.md only describe internal paths (gh sparse-clone script, manual copy from gim-home), so external contributors who clone the public repo and try to work on the analyzer have no obvious way to obtain the rule zips.

Additionally, CONTRIBUTING.md does not mention rule-zip setup at all, so analyzer contributors get silent "unknown operator" warnings without knowing why.

Proposed fix

Rule zips are already published as individual assets on the public WinML-ModelKit releases page. Document this as the recommended path for external contributors:

1. Add a new "Option 1: Download from the latest GitHub release (for external contributors)" to runtime_check_rules/README.md, using gh release download --pattern '*.zip'. Demote the existing gim-home-based options to "Microsoft internal".
2. Add a "Runtime check rules (for analyze development)" subsection to CONTRIBUTING.md that points external contributors at the release-download path and links to the README for full options.
3. Update scripts/download_rules.py docstring and "missing account" error message to redirect external users to the release download.