Add separate Raster analysis articles for Julia and R and replace the three language article by the introduction

This will hopefully render nicer than the three language article. I added the metadata block from the main article to the sub articles as well. The pipeline currently fails, but I don't know whether that is a problem from my changes. This should close #182 (closed) for now, but I would still like to be able to have articles that show multiple languages side by side.