Replace Quick Start in Docs with SphinxGallery

[wtbarnes]

Currently, the Quick Start sections of the docs rely on static pngs and code snippets that need to be updated and uploaded manually. As a result, they easily fall out of date (e.g. the PNGs in the docs are still leftover from the old SourceForge docs).

Instead, we should use the matplotlib plot directive such that the code snippets are always up to date and produce the plots in line each time the docs are generated. This ensures that the included plots are not out of date with the code itself. As an example, see how SunPy has implemented this. It is pretty straightforward.

Additionally, we should use sphinx-gallery to provide some nice standalone examples of the capabilities in ChiantiPy and to provide links to downloadable notebooks and scripts. As an example, see the SunPy docs.

(This will probably be a rather lengthy project so if there is a consensus here, I would suggest moving this to our list of long term projects in the wiki.)

[kdere]

Although I have not looked at any of the examples, it sounds like a good idea. The static pngs are often out of date and hard to manage. I will let you add this to our long term projects. I am currently busy upgrading the lithium-like sequence of CHIANTI, in particular, the satellite lines.