`book`: Document probe-run json mode
- [panic! and assert!](./panic.md)
- [Printers](./printers.md)
- [Encoding](./encoding.md)
+ - [JSON output](./json-output.md)
- [Migrating from `v0.2.x` to `v0.3.0`](./migration-02-03.md)
- [Design & impl details](./design.md)
- [Interning](./interning.md)
