chore: Move derivation of df_complete into _setup.qmd

[ns-rse]

• Key dataframe needs to be available across all work, makes sense to have it created early on rather than part way through manuscript.
• Adds some better summary tables of missing data (there is actually quite a lot).
• Tried to add a visualisaiton of missing data using visdat::vis_miss() but it prevents the document from rendering correctly. Image can be seen in this issue though.