Update docs page sidebar for "Epics" tooltip to show premium/silver
Problem to solve
Epics moved down a tier to premium/silver from Gold/Ultimate, but the tooltip did not get updated to reflect that (the badge does, however).
In addition, Per slack, @marcia also mentioned: "We need to update not only the nav, but also say in the docs that the feature was moved from Ultimate to Premium in GitLab X.Y."