Add pubCommonId optout example

[pycnvr]

Update the opt-out instruction and add an example.

[bretg]

@pycnvr - thanks for the doc update, but note that the PubCommon ID module is scheduled to be deprecated soon - January. We're re-building the pubcommon functionality in a new "Universal ID" module as described in prebid/Prebid.js#3046

I think this means we should be adding an optout cookie to the new module. That's a little redundant with the GDPR support we're already building in, but is easy enough.

[bretg]

Confirmed that _pubcid_optout is already in the module. Merging.

[pycnvr]

Thanks. You are right. Most sites will probably go with GDPR once the universal module is available, and this is just another option in case GDPR isn't used yet.

[goosemanjack]

This brings up a good point - namely that the UniversalID integration needs to pub/sub to all integrated ID systems so that those system can sync the opt-out settings.