Documentation: show examples of using htmlDependency

[daattali]

htmlDependency() is an important function for shiny app developers, yet it's not very easy to understand how to use it by reading the documentation. The documentation is technically complete and has all the required information, but providing a few examples would help a lot.

I personally never remember how to use it and every time I need to use it I end up looking at previous usages, and (anecdotally) from speaking to others that seems to be a common theme.