Add example gallery

[dstansby]

It would be great if an example gallery could be added to the docs, with a very simple example showing how to load a file (perhaps from IRIS?) into sunraster.

[nabobalis]

There are examples in irispy on this now as that IO code is there now.

Adding an example for SPICE would be good.

[hayesla]

just building on this, currently at the Solar Orbiter summer school, and I think it would be really nice to show an example of reading in some SPICE data, pulling out a spectral line for example, and maybe fitting it with astropy etc - ping @AlexandrosKoukras. we could also include how to get data from SOAR with sunpy_soar

[AlexandrosKoukras]

There is already an example on how to read SPICE data in the SPICE website which could be a nice starting point.