Document aspectmode for 3D axes in example page

[jonmmease]

Suggestion by @chrisranderson moved from plotly/plotly.py#1228

I'm plotting spatial data and I wanted the aspect ratio of the data to be preserved, but I couldn't figure out how. I eventually found plotly/plotly.py#70 and aspectmode=data.

In https://plot.ly/python/3d-axes/, there's no mention of "scale", "ratio", or "aspect" when I ctrl+F through. I think it would be helpful to add an example showing 1) skewed data due to the cube default and 2) data with square angles after using aspectmode=data. That seems like the most natural place to demo this functionality.

Thanks!