Add alt text for images included in the lesson for improved accessibility

[mellamosummer]

Many of the images used in the lessons do not have descriptive alt text.
For example, at https://datacarpentry.org/wrangling-genomics/04-variant_calling/index.html the following image is presented:

The alt text for this image only states "workflow_align"

This can be improved for users that may use screen readers by saying something along the lines of: "a flowchart with 3 steps is presented. The first step is 'sequence reads', the second step is 'Quality control', and the third step is 'Alignment to genome'. FastQ is the file format for steps 1 and 2. SAM/BAM is the file format for step 3. "