clearly identify the containing directory for every filename_* config option

[cjyetman]

For all the filename_* type config options documented in https://rmi-pacta.github.io/pacta.multi.loanbook/articles/config_yml.html, we should clearly indicate the directory/path it should be found in, which currently can only be what's defined by either dir_input or dir_output (I think). Previously, it was a bit more obvious from the config option names, e.g. filename_abcd was obviously in dir_abcd, but now that we've consolidated everything into inputs and outputs we should be more clear about it in the documentation. Additionally, there are still some sub-directories that get created / are expected, e.g. outputs/matched_loanbooks, so we should be cautious to identify if certain filename_* type config options define the name of a file that actually lives in one of those sub-directories.