Declare the 'markdown' package as a dependency

Piplines failed during the build:

The 'markdown' package should be declared as a dependency of the 'airGRdatassim' package (e.g., in the 'Suggests' field of DESCRIPTION), because the latter contains vignette(s) built with the 'markdown' package. Please see https://github.com/yihui/knitr/issues/1864 for more information.