add example data

[youngroklee-ml]

• Add example data from https://academic.oup.com/ije/article/49/5/1719/5835351 as lungcancer.rda inside data/ folder
• Create data documentation with as roxygen2 block in R/data.R
• Compile package and check data load and data documentation

Relevant material: https://r-pkgs.org/data.html#sec-data-data