i18n for servant-docs labels

[ciukstar]

I looked at the source code and saw that messages like "Supported content types are" and "Response body as below" are hardcoded.
Please add i18n for labels if possible.
Thanks.

[tchoutri]

@ciukstar Hi! At the moment, servant-docs is lacking ownership. If you wish to have a go at the implementation you would be most welcome. :)