Cleanup and reorganize "What's new" for v0.10
Cleanup and reorganize "What's new" for v0.10

I would particularly appreciate feedback on the "Highlights" section. Is
anything else particularly worth calling out?

markelg/xray,pydata/xarray,pydata/xarray,jcmgray/xarray,markelg/xray,shoyer/xarray,markelg/xray,rabernat/xray,pydata/xarray,jhamman/xarray,chunweiyuan/xarray,jhamman/xray,jhamman/xarray,xray/xray,jhamman/xarray,shoyer/xray,shoyer/xarray,rabernat/xray