Support export to dash docs format

[samaloney]

The dash docs format is supported by a number of offline/enhanced documentation browsers (e.g dash, zeal) it would be nice to have this option for Sunpy.

The tool to create the documentation sets supports sphinx out of the box and I was able to get a decent looking version by hacking my local version of the theme deleting the contents of navbar.html and docsidebar.html.

From my experimenting four major things need to be done:

• Remove or make top navbar scroll with page
• Remove sidebar
• Include guides section in application TOC
• Include example gallery in application TOC

I think the first two could by handled by having a different set of templates and changing that with a command line arg. The last two I'm not sure about need to see what Matplotlib does for example.