Add societal impact introduction

[vtraag]

This add a general introduction to the Societal Impact section.

[vtraag]

Thanks @tommv, this is quite extensive as an introduction! In general, I think it makes quite a nice introduction.

However, there's one aspect missing still, I think, namely the scope. That is, I think it would be good to explain why only certain indicators are included (mostly on uptake) and not others. As I recall our earlier conversations, our arguments were something like the following: There are thousands of societal issues that are studied across the social sciences (e.g. income/wealth inequality, segregation, urbanisation, demographics), but we are not in a position to say something specifically informative about all of these thousands of indicators. In addition, for these issues it is very difficult to establish any influence of science at all (irrespective of it being open or not), and so one way of trying to limit the scope of the handbook is to only consider those areas where we have some hope of tracing something back to science still, and this is what is included in the handbook (except for the more general "societal issues", as you already point out). Could you include something along those lines?