Add a Sphinx plot gallery?

@jberlanga, @adrn, all – What do you think about adding a sphinx plot gallery to the `astroplan` docs.
I haven't used this, but I think it's very easy to do:
http://sphinx-gallery.readthedocs.org/en/latest/auto_examples/index.html#examples-index

I wasn't sure if it's a good idea at all, and if so it's probably for later (although it could just as well be set up now), so I made a separate issue from #7 for discussion ...

Start by reviewing the current astroplan documentation and the linked Sphinx Gallery examples-index; the issue names no repository files or tests. Confirm whether a gallery is wanted and, if so, define the documentation entry point and acceptance criteria before implementation.

Written by the indexing model from the issue text.