Interactive visual components in Part 3 of the guide

[AkshitaB]

The old guide had React components that highlighted information about what the code was doing, when hovering on a line. Eg. https://github.com/allenai/allennlp-website/blob/5f4b7aeb763e05602415da109e58f61aed4c591d/_includes/default-tutorial.html

This visual style is useful when learning by following a tutorial. We can do the same for Part 3 tutorials. Eg. on hovering over a line that uses the Trainer, we provide a short description of what Trainer does, and also add a link to the relevant modules in Part 1 and part 2 of the guide (or docs) for more details.

This will require having reusable React components.