Add {roxygen2} documentation to application logic

By convention, modules in a {golem} app are contained within R/mod_*.R scripts. For this app, these are separated into R/mod_*_ui.R, R/mod_*_server.R. These functions should have {roxygen2} documentation added so we know what each module does and what the inputs are to each server, in particular.

Note that there are some special cases of R/mod_*_utils.R, which generally contain functions that are run within the corresponding R/mod_*_server.R functions (perhaps arguably these should be in their own R/fct_*.R functions instead, in which case the documentation could be handled in #406).

Note also that there are some small functions used on the fly withon modules that could in theory be added to the relevant (or new) R/mod_*_utils.R script, or in an R/fct_*.R script.

Start by inventorying the R/mod_ui.R and R/modserver.R scripts, then read each module's functions and inputs. Check the related R/mod*_utils.R files for special cases and decide whether their functions belong with the module documentation. Done means the module and server inputs are documented with roxygen2 across the relevant application-logic files.