Clippy Book Chapter Updates Reborn: Macro Expansions
This PR adds a new chapter to the book: "Macro Expansions". There weren't big changes apart from grammar, re-phrasing and stylistic choices.
## Notes
- **Does not require any other chapter** to be merged before this
- To talk about the whole project, please use the tracking issue for the project #10597 (It also contains a timeline, discussions and more information)
changelog: Add a new "Macro Expansions" chapter to the book
r? `@flip1995`
90cb0fa2f3