[Docs] show off tag/flag distributions in /product/issues/issue-details/

See https://docs.sentry.io/product/issues/issue-details/

High level explanation of tags/flags drawer features - what it is, how to access it. For flags, link [eval tracking details](https://docs.sentry.io/product/issues/issue-details/feature-flags/#set-up-evaluation-tracking)

Link issues team on the PR, not sure if this page includes new-UI-only features yet.

 