Add Path field to Proposal for accurate KEP README.md URLs

KEPs under SIG subdirectories (e.g. sig-cloud-provider/azure/,
sig-cluster-lifecycle/kubeadm/) produce incorrect links in keps.json
because owningSig does not include the subdirectory.

This adds a Path field to api.Proposal containing the actual filesystem
path relative to keps/, enabling downstream consumers to construct
correct README URLs.