Add ERR to ranking evaluation documentation (#32314)
Add ERR to ranking evaluation documentation (#32314)

This change adds a section about the Expected Reciprocal Rank metric (ERR) to
the Ranking Evaluation documentation.
vroyer/elassandra,vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra