Document public data release governance

[fcorowe]

Summary

• adds CODEOWNERS, PR template, and contribution guidance for PR-only data governance
• updates public install instructions for debiasRdata
• records resolved coverage_lad license, provenance, and aggregate/privacy release notes in source docs and rebuild metadata
• clarifies package code/docs MIT license plus source-specific data licenses

Validation

• Rscript -e "devtools::load_all('.', quiet = TRUE); testthat::test_dir('tests/testthat', reporter = 'summary')" passed
• secret/raw-file scan found no credentials and confirmed raw source inputs remain ignored

Notes

This PR should be reviewed and merged before making de-bias/debiasRdata public.