feat: Load modules, setup R env, tidy cleaning script, setup data

Initial work on Sheffield cohort.

Copies shf_thy_nod.R from thyroid-cancer-prediction repository and tidies it up
Adds document structure with some text on some sections and lots of TODO sections where work needs adding
Loads and cleans data using cleaned r/shf_thy_nod.R
Basic tables detailing variables
Summary table of data completeness (some labels not yet carrying through properly)
Subsets data and sets up training and tests subsets and defines the "recipe" for the Tidymodels framework
Adds some citations for some of the statistical methods

ns-rse / sheffield-thyroid