Add validation for documentation code examples

## Context

Ensure that code examples in documentation (especially in testing.md and other docs) compile correctly. This can be achieved either by:
1. Adding a CodeRabbit rule to validate examples
2. Externalizing examples from documentation files so they can be tested as part of the build process

Reference implementation exists in the typeberry project.

## Related
- PR: https://github.com/FluffyLabs/jammin/pull/106
- Comment: https://github.com/FluffyLabs/jammin/pull/106#discussion_r2811545831
- Requested by: @tomusdrw