Expand "Purpose of this Analysis"

Looking across the notebooks, the “Purpose of the analysis” section is usually a single sentence, and does not usually give any "why". For example, the for PCA for 02-microarray/dimension-reduction_microarray_01_pca.html, it reads:

> This notebook illustrates one way that you can use microarray data from refine.bio to perform Principal Component Analysis (PCA) and plot the scores using ggplot2.

I think it would be helpful to expand many of these by a sentence or two. In this example, we might say something brief about what PCA is doing and how it is useful (maybe even how it is not).

Start with 02-microarray/dimension-reduction_microarray_01_pca.html and review the existing “Purpose of the analysis” section. Compare that wording with the corresponding sections across the notebooks, then expand the selected sections with brief context about why each analysis is useful. Done means the relevant purpose statements explain both what the analysis does and why it is used.

Written by the indexing model from the issue text.