Move Cohorts out of instance statistics

Overview

With #196455 (closed) and #196454 (closed), we can start hiding Instance Statistics in the top navbar when this feature's admin-only.

We're not confident that Instance Statistics, as currently displayed in the navbar, is the right place for these features to thrive. We should consider moving them elsewhere or refactor the Instance Statistics area into a more useful tool.

Proposal

• Move Cohorts to the admin panel (#233833 (closed)).
• Update the documentation for how to navigate to Cohorts.