Improve example dataset and docs

[cdeil]

This pull request:

• Adds a nice example dataset for docs and tests
• Improves and extends the getting started page to use the new example dataset
• Moves scripts to profile regions outside the package

At first I wanted to use the real ExampleFermiDataset and bundle a few 100 kB of example data in the repo.
But then I thought that this would be problematic when moving this to Astropy core, so I started to write code to simulate some example data.
I can still add the Fermi-LAT example dataset if others think that would be better.

This is work in progress ... I'll try to finish it tonight.

[cdeil]

I'm merging this now. Will continue in other PRs.