Add sphinx-gallery

[Astro-Kirsty]

This is a first attempt at adjusting to use sphinx gallery. It was so hard to create a minimal PR as so many files will change!!!
We will have to remove many things

example.py ← Sphinx-Gallery input
example.ipynb ← optional executed notebook for download

Okay, so we want a .py file for each recipe.
Then we can pre-execute the .py with the .ipynb

I started here by adjusting the css and implementing the .py files with:
python /gammapy/dev/ipynb_to_gallery.py background_model.ipynb

The gallery looks like this currently;

The individual recipes have not been executed yet, so they do not show the correct content.

[review-notebook-app]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

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