add docs related to URL query strings for external studies

[becky-gilbert]

I don't think the docs mention the fact that URL query parameters are added to external study links, so that researchers can automatically get/record child IDs on their external study platform (Qualtrics, etc.). Please let me know if I've missed this somewhere! If not we should add it.

[mekline]

Reminder/comment that there are three cases of "link exit" from Lookit, all of which should ideally work in the same way -

• The "Exit URL" parameter on an internal study (study redirects there after the end of a study)
• The study link parameter on an external study
• A link manually added to an internal study at the end that participants click on (I may be wrong about whether this case adds query strings though...) <- This provides similar functionality to the first but I know it's a pattern some researchers use...

[becky-gilbert]

Thanks @mekline! I had only been thinking about link parameters for external study types, so this is very helpful 👍