Chapter 7: Mention design documentation

[GBKS]

Chapter 7 goes into the practical assets designers create for projects, but it doesn't mention documentation. It is very helpful to document the process and create an explicit design system that is not just baked into the code, but also has its own place (like what we have for the core app).

We could explain the importance of doing this, and also how to slowly build it up over time (instead of trying to do a massive documentation project that is outdated as soon as its done).