UID2 module doc doesn't cover client-side integration

[bretg]

Our solutions team found the hard way that the Prebid document for the UID2 user id module is only for the UID2 Client-Server Integration Guide for Prebid.js

Apparently there's another type of integration described here -- UID2 Client-Side Integration Guide for Prebid.js

They've figured out how to implement the 'Client-Side Integration', but the Prebid docs ought to be updated to explain the two different methods.

Adding @lionell-pack-ttd, @jingyi-gao-ttd, @ssundahlTTD, @jdwieland8282