Update model_grid.rst
Update model_grid.rst

grouped the intro bullet points into two sections
landlab/landlab,ManuSchmi88/landlab,Carralex/landlab,amandersillinois/landlab,cmshobe/landlab,cmshobe/landlab,ManuSchmi88/landlab,cmshobe/landlab,SiccarPoint/landlab,Carralex/landlab,RondaStrauch/landlab,laijingtao/landlab,ManuSchmi88/landlab,RondaStrauch/landlab,csherwood-usgs/landlab,decvalts/landlab,laijingtao/landlab,amandersillinois/landlab,SiccarPoint/landlab,RondaStrauch/landlab,landlab/landlab,Carralex/landlab,decvalts/landlab,landlab/landlab,csherwood-usgs/landlab