Add link to Shiny tutorial in R user guide

The only change in content is adding a line (cell) under section "Embedding" (which I renamed "Embedding in web pages" if that's all right).  Really that's where Shiny belongs; it's about embedding an iframe in HTML.  And it really makes sense to have the link ("https://plot.ly/r/shiny-tutorial/") at the same level as "https://plot.ly/r/embedding-plotly-graphs-in-HTML/".
The other changes come from automated processing.

Relates to https://github.com/plotly/streambed/pull/598
@etpinard @pedrodz @msund