Replace Binder in README with Google Colab

[jeffjennings]

Also rewrites the README a bit to be shorter.

Closes #615
Closes #616
Closes #535

• Check the box to confirm that you are familiar with the contributing guidelines and/or indicate (check the box) that you are familiar with our contributing workflow.
• Confirm that any contributed tutorials contain a complete Introduction which includes an Author list, Learning Goals, Keywords, Companion Content (if applicable), and a Summary.
• Check the box to confirm that you are familiar with the Astropy community code of conduct and you agree to follow the CoC.

[adrn]

We went with #617 instead!