Case studies are expected to live in inst/case_studies and to be named
using the pattern CS<chapter>_<number>.Rmd (e.g. CS9_2.Rmd).
The table has two columns: File (the case study identifier) and
Title (extracted from the YAML header). Case studies are listed in
numerical order, not alphabetical order.
The function invisibly returns a character vector of case study identifiers.