Preface/index notes

[annakfell]

• [ ] Data - The link to https://norcalbiostat.github.io/pma6_code/ in the preface works but links to each PMA6 dataset and codebook on the page give a GitHub Pages 404 error so they aren't downloadable - https://www.norcalbiostat.com/data/ links work for depression, lung function, and parental HIV, but not sure if any other PMA6 datasets are used later on that are missing from this page
• [ ] Data - This section could also include introductions to the datasets used throughout the notes, or this could become a separate unnumbered section after it
• [ ] Acknowledgements - @annakfell write what you worked on