Alternative text of images

[Czirion]

Reviewer's comments:
1)The image https://carpentries-incubator.github.io/shell-metagenomics/fig/02-01-01.png does not have a sufficiently detailed alt text.
2)The image https://carpentries-incubator.github.io/shell-metagenomics/fig/02-02-01.svg does not have a sufficiently detailed alt text.
3)The alt text of image https://carpentries-incubator.github.io/shell-metagenomics/fig/02-05-01.png could be slightly more descriptive.
4)The image https://user-images.githubusercontent.com/67386612/118720027-ba433c00-b7ee-11eb-87e5-7496fde5763e.png should have a more detailed alt text (that being said, the Figure 1 caption should be sufficient for this)
5)The alt text of image https://carpentries-incubator.github.io/metagenomics/fig/03-02-03.png has some phrasing issues, and could also be enhanced.
6)The alt text of image https://carpentries-incubator.github.io/metagenomics/fig/03-03-01.png could be enriched with more information.
7)There is insufficient alt text in the image https://carpentries-incubator.github.io/metagenomics/fig/03-04-01.png and https://carpentries-incubator.github.io/metagenomics/fig/03-05-01.png
8)There is insufficient alt text in the image https://carpentries-incubator.github.io/metagenomics/fig/03-06-01.png

To do: Improve alternative text in the mentioned images following the alternative text guidelines: UCSF: Accessibile Images Best Practices
The bIg Hack: Avoid these common alt-text mistakes.
And review the alternative text in the rest of the images not mentioned here.

[Czirion]

1. done
2. done
3. done
4. done
   5)done

[Czirion]

6. done
   7)done
   8)done