Regarding those old sources for the Haskell 2010 Report: the search term used was "Haskell 2010" latex sources.
If you choose to work on your own private draft/prototype version, do try to work with the Tex documents directly - Tex for publishing book and papers is like HTML for publishing to the Web: it’s ubiquitous. If you look, there are probably Tex convertors for almost every other format, including mdBook and sphinx.