improve mobile experience

[maxheld83]

In the daily call we just wondered about the mobile experience @njahn82 @Henrieke72.

• [x] The pkgdown part of the website is just plain bootstrap, so that should be pretty responsive.
• [x] The iframe (shinyapps.io) part is currently flexdashboard, which is also bootstrap and reflows to scrolling behavior quite nicely
• [ ] the plotly plots cause a typical problem, where a touch scroll is interpreted as a mousedown event, and so it's iffy to scroll down.

I'll investigate this more down the road.

But if you run into anything else that's weird @njahn82 and @Henrieke72, feel free to log this here.