Rewrite "why cubed?" for users

[TomNicholas]

The current text on why cubed? is written at a very high level. It's very interesting for people who think about different distributed systems, but it's not super clear to new users who just want to know what cubed can do for them.

Perhaps we should move the existing text to e.g. design/motivation, and rewrite this intro text to instead emphasise:

• cubed is a drop-in replacement for dask.array
• it tells you if your computation would run out of memory
• it is designed to be very robust to failures, and be resumable
• it doesn't require you to make many (if any) up-front choices about configuration / cluster size
• (Optional) IF you want to run in the cloud it is very horizontally scalable and stateless / serverless