Add preface to proceedings

[HKervadec]

Two things I was not sure how to fill the metadata:

1. first and last pages, as the preface is with roman numerals (first paper starts at 1)
2. order, that I left at 1. Or should another commit shift https://github.com/mlresearch/v227/blob/gh-pages/_posts/2024-01-23-donteu24a.md to let's say, 2?

[lawrennd]

Struggling to remember precisely myself, but I think the changes I've made work ... if not we can look again. I'll integrate these.