Add validation for documentation code examples

[coderabbitai]

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: Implements TestJam, Testing helpers and Testing docs #106
• Comment: Implements TestJam, Testing helpers and Testing docs #106 (comment)
• Requested by: @tomusdrw