Text supplementation for content-heavy figures

In [adding alt-text to our images](https://github.com/carpentries/instructor-training/pull/1196), @ErinBecker noted:

> In particular, the two images in 21-carpentries.md (a timeline and a Venn diagram) were challenging, as was the Bloom's taxonomy diagram. We should probably have that information summarized in text form in the lesson body, not just in alt text.

Start with 21-carpentries.md, where the timeline and Venn diagram are mentioned, and also locate the Bloom's taxonomy diagram. Read the surrounding lesson text and add text summaries of the information conveyed by those images. Done means the lesson body provides the information without relying only on alt text.

Written by the indexing model from the issue text.