Improve documentation on backends

[OriolAbril]

Improve backend related docs, especially none backend. Or should we name it "data" backend as no actual drawing/rendering happens, only defining which data goes into which plotting function?

We probably want to generate a "logo image" for it so https://arviz-plots.readthedocs.io/en/latest/api/backend/index.html doesn't look so bad. Naming it "data" would kind of match the "logo" idea being used currently.

[aloctavodia]

I am not opposed to "data" but none is fine to me. The logo could be a an "eye" icon inside a "none" icon. Or just an "eye" icon with a line crossing it.