[manuscript] Add Supplement S4: description of the Nepal SEES typhoid use-case dataset

[Kwan-Jenny]

Add a supplementary section that describes the nepal_sees dataset used in
the Use section of the manuscript. This should parallel Supplement S4
of the serocalculator paper. It should include:

1. Brief background on the Surveillance for Enteric Fever in Asia Project
   (SEAP) and the SeroEpidemiology and Environmental Surveillance (SEES)
   sub-study (Aiemjoy et al. 2022, Garrett et al. 2022).
2. Inclusion criteria for the longitudinal arm: blood-culture-confirmed
   enteric fever cases enrolled in Nepal.
3. Sample size: n = 145 culture-confirmed subjects, 689 longitudinal
   HlyE IgG/IgA observations (the subset shipped as nepal_sees).
4. Specimen schedule: baseline, 28 days, 3 months, 6 months, 12 months,
   18 months as available.
5. Laboratory methods: ELISA for HlyE IgG and IgA.
6. Reference to the data dictionary and CSV shipped as
   inst/extdata/SEES_Case_Nepal_ForSeroKinetics_02-13-2025.csv.

Referenced in manuscript at: Use section:

"Additional details on these data are available in Supplement [TODO]."

Once written, replace the [TODO] in the manuscript with a reference to
the supplement (e.g. "Supplement S4").