Improve drill-down section

[cpsievert]

It should take more time to build the core concepts, like http://bit.ly/jj-shiny-day does

Also, take advantage of plotly/plotly.R#1655, when fixed

[cpsievert]

Also, maybe "Advanced applications", should be renamed to "In practice", and some lighter topics/examples should be added to the beginning (e.g., linked brushing, relative frequencies, etc)