Add Mucho Marker example to docs

- fig : https://plot.ly/~etpinard/435/
- mucho marker : https://github.com/plotly/streambed/pull/358
another solution might be to include an example with customized *per -trace* properties, e.g. color, symbol, and then include a "prepend" or "append" (cc @enginear, @alex-vados) note that has a link to this example, "to see all possible marker types"