[r] Add examples for single cell plots and trackplots

[immanuelazn]

This PR provides function examples for most of the functions in the Single Cell plots and Genomic track plots section.
This PR also provides a function (render_plot_from_storage()) that allows for resizing plots in an example, as pkgdown does not natively provide configuration on that end.

Missing functions include the following:

• plot_tf_footprint()
  • This requires a lot of setup infra and I don't see an easy way of generating fake data that makes sense for these footprints. Should I just go ahead and add in the data preperation code anyways?
• set_trackplot_label() set_trackplot_height() and get_trackplot_height().
  • I think all of these functions are dependent on the trackplot_combine() function. This would turn into an entire mini-vignette with customizing the trackplot_combine() plot. Should this be the goal?

There are some various thoughts, and issues with the plots, that I will note directly in code. However, there is a large issue that likely comes from argument_name() that causes the plots in pkgdown to be different than what is rendered in console. I tried with and without rendering to storage, but the issue still persists.