Cross-link to bootstrap_with_dense_weekly/ + top-level substrate SCHEMA

Cross-link to bootstrap_with_dense_weekly/ sibling

Add/update imputation bootstrap reference: draws.meta.json

Add/update imputation bootstrap reference: draws.parquet

Add imputation/bootstrap/SCHEMA.md

Add imputation/bootstrap/README.md

Drop redundant pooled per_user_errors meta

Drop redundant pooled per_user_errors (concat of per-method files)

Add/update imputation bootstrap reference: per_user_errors.meta.json

Add/update imputation bootstrap reference: per_user_errors.parquet

Add/update imputation bootstrap reference: draws.meta.json

Add/update imputation bootstrap reference: draws.parquet