add analytics for docs

[krancour]

@zr-msft suggested that this would be useful for gaining insight into what bits of documentation are valuable or not, dead ends, pain points for users, etc.

[krancour]

Just an update on this. Everything for this was actually already in place on the site, but some further effort is still required on the GA end to start extracting actionable intelligence from the raw data.

I'm going to assign this to myself.