Example on front page hard to follow

[dstansby]

I am coming to gwcs as a first time user, and thought the first two narrative sections on the documentation page (https://gwcs.readthedocs.io/en/latest) were a great introduction to gwcs.

I found the example in the third section (A step by step example of constructing an imaging GWCS object.) hard to follow though, because it is focused on precisely replicating a fitswcs, instead of explaining the concepts and API of constructing a gwcs.

My suggestion for improvement would be to move this example out of the introductory section, and replace it with a simple example that uses only one transformation step, but still shows how the gwcs API works in practice.

[perrygreenfield]

Addressed by #483