Improve aesthetics of QC report

[jashapiro]

The original vision of the sample QC report was a two column layout (shown belwo), but this will probably require a bit of thought and design work. Some of this work falls under #508, but we might also want to consider a broader look at aesthetics, maybe even to fit with the CCDL/ALSF style.

In working on a trial run of #508, I did confirm that adding a custom .css file in the same directory as the .rmd should work as a means to accomplish most aesthetic adjustments.

Originally posted by @allyhawkins in AlexsLemonade/scpcaTools#37 (comment)

[sjspielman]

Noting I'm sending this issue over to scpca-nf, which is where the QC report now lives.