Runnable and testable code snippets

[niekdt]

Code snippets would be a lot more reliable if they are defined to be runnable and testable.

Possibilities:

• More complete code examples for users, even showing output
• Flagging broken snippets
• Automatic supported-package version labeling
• Version-specific views

However, this proposal greatly increases the amount of effort per code snippet, so this only becomes feasible with more support from other contributors.