Adding to the book

Nobody has claimed this yet.

Hi, there are some parts of the book that I think don't need a druid expert in order to write. Would it be fine to leave this issue here and track what parts need help or entire additions? I can probably add a task list.

So this is a rough task list split by chapters:

### Getting Started With Druid
- [ ] Finish `Application state` section
- [ ] Write `Handle user input` section
- [ ] Write `Putting it all together` section

### Localization
- [ ] I don't know what needs to be done here.

### Command
- [ ] I also don't know what need to be done here.

### Widgets in Depth
- [ ] Layout
- how constraints work
- child widgets and `set_layout_rect`
- paint bounds
- [ ] container widgets
- [ ] widgetpod & architecture
- [ ] commands and widgetid
- [ ] focus, active, hot, (disabled when it gets there)
- [ ] request paint & request layout
- [ ] changing widgets at runtime
### More information
- [ ] Add more to the projects using druid ( There are a few new projects in #1360 )

Start by reviewing the book's existing structure and the listed chapters, especially Getting Started With Druid and Widgets in Depth. Select a specific section before beginning, since several entries are broad or explicitly undefined. Done means the selected sections or additions are written and the task list accurately reflects their status.

Written by the indexing model from the issue text.