Simple but imperfect fix for xray candidates race causing /explore to not show sample dataset candidates

Add paragraph about x-ray permissions

Fix `Unauthenticated` nested query test failures

Docs for 0.29 (WIP)

x-ray titles redux

fix drill-through to automatic dashboards

Ensure the spected HTTP status code is included in
`metabase.query-processor-test.nested-queries-test` POSTs as when it
is not included our `retry` unauthenticated logic doesn't get
executed. Depending on the test order, the database and mock session
store might be out of sync and not retry authentication failures.

Fixes #7490

enable question xrays

0.29 Instrumentation

Usage stats for Alerts & CSV/XLS Pulses

xray: add /:prefix/:rule endpoint for saved questions and ad-hoc questions

Less aggressive entity type inference

xray/nested questions