Current the source code is prepared to have different shading & contour data per country - see ShadingAndContours.tsx. If you only have data for other countries or a single region then update that file accordingly.
For global DEM data ALOS World 3D - 30m seems to be currently the best source. First you must create an account there to be able to download DEM data.
To create hillshading use Makefile. Depending on the input data it may need some customization. For contours see DIRTY-NOTES.md.
TODO: improve documentation
Rendered shading files are stored in shading/{cc} directory, where {cc} is a country code of a shading specific to a particular country. In every counrty folter there must fe a final.tif file and mark.tif. Directly in the shading is also final.tiff with "global" shading - a fallback shading with the lowest reslution.