In Document[key] / Document::getField(key), we have this code:

    template <typename T>
    Value operator[](T key) const {
        return getField(key);
    }
    template <typename T>
    Value getField(T key) const {
        return storage().getField(key);
    }

This is creating copies of key, which could be an issue when called with expensive-to-copy T values like std::string
I added some static_assert(!std::is_same_v<T, std::string>); for fun in there, and then compilation fails. This means we are calling these functions with std::string arguments from some call sites.
We should fix this to avoid any unnecessary string copies here.