Create guide-level documentation / a Taffy "book"

## What problem does this solve or what need does it fill?

People often seem confused about how to approach / get started with using Taffy. And while Taffy has good rustdoc coverage, and a few basic examples, it doesn't really have good guide-level documentation.

## What solution would you like?

Creation of guide-level documentation. Something like

- https://dioxuslabs.com/learn/0.4/
- https://serde.rs/
- https://femtovg.github.io/

## What alternative(s) have you considered?

- Try to add this information to the README / docs.rs docs.
- Create a more involved / well-documented example (we could probably do this as well?)