Add individual panel labels for Figure 2 QC report examples

[jaclyn-taroni]

I was reviewing AlexsLemonade/ScPCA-manuscript#36, and I agree with @sjspielman's comment:

While writing this, I wondered whether we actually want to make these real panels in their own right, as panels B-G, which may make it easier to read.

[sjspielman]

I also think S2B would benefit from this (thumbs up if you agree?) - https://github.com/AlexsLemonade/scpca-paper-figures/blob/main/figures/compiled_figures/pngs/figure_s2.png

Potentially also the whole of S1, but I think this one is less problematic - https://github.com/AlexsLemonade/scpca-paper-figures/blob/main/figures/compiled_figures/pngs/figure_s1.png

[jaclyn-taroni]

S1 seems fine as is to me