Add Mucho Marker example to docs

- fig : https://plot.ly/~etpinard/435/
- mucho marker : https://github.com/plotly/streambed/pull/358
is it confusing that this will be peoples first introduction to setting arrays for attributes inside a trace? previously, we introduced this concept in the bubble charts section. people *might* think that to set a trace to square symbols , you have to set a symbol array, and set *every* single element in that array to "square". .. you know what I mean?