doc: show multi concat in lang overview
doc: show multi concat in lang overview

diku-dk/futhark,diku-dk/futhark,mrakgr/futhark,ihc/futhark,ihc/futhark,diku-dk/futhark,mrakgr/futhark,HIPERFIT/futhark,mrakgr/futhark,diku-dk/futhark,HIPERFIT/futhark,ihc/futhark,HIPERFIT/futhark,diku-dk/futhark,CulpaBS/wbBach