enclose YAML strings containing % with quotes

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4029

Thanks @iamdto, did you spot some other occurences in the docs?
Well, we don't have that much solutions:
 1. We fix the syntax of all JSON examples in the documentation (though it could happen on other languages too).
 2. We had back the light on dark theme for the default syntax highlighting.
 3. We keep it like this.