Update "Design & impl" section in book.

[Urhengulas]

The section is out of date in many parts. It should be revisited to make sure it reflects the current design and implementation.

https://defmt.ferrous-systems.com/design

For the moment it would be good to add a header to every page which warns about it being potentially outdated. Maybe > ⚠️ The design and implementation chapter is outdated ⚠️.

[BenFordTytherington]

Warnings added in #912

[jonathanpallant]

OK, now we have the warnings, we really need to go through and update the book and remove the warnings :)