Rewrite Cubed intro docs

[tomwhite]

Here's my first attempt at #565. Appreciate any thoughts you may have @TomNicholas and @rabernat.

[tomwhite]

From @TomNicholas in conda-forge/cubed-feedstock#18 (review)

Probably the wrong place to discuss this, but I'm torn on "scalable" vs "bounded-memory". I feel like the latter is actually the key concrete innovation of cubed, which the former (hopefully) follows from.

I'm not sure either. Maybe we just need both? Although I was trying to make it shorter.

[TomNicholas]

On the tagline:

I think you need to say "bounded-memory" as that is really the USP, and I think you need at least one other word to convey the fact cubed can scale. That could potentially be "scalable", "out-of-core", or "distributed". I don't have a strong opinion beyond that, but I think you need both of those to communicate what cubed offers.

On the rest of the layout: We discussed this in the meet last week and I think it looks great! I think the 6 main boxes are well-chosen features to highlight.

[tomwhite]

Thanks Tom - excellent points.

I'm thinking of updating the tagline to "Scalable array processing with bounded memory".