Docs: Document Prometheus-Alertmanager integration settings

[gguillotte-grafana]

What this PR does / why we need it:

Documents alertmanagerUid and manageAlerts for the Prometheus data source, and implementation for the Alertmanager data source. These were implemented without documentation around Grafana v8.3.

Which issue(s) this PR fixes:

Fixes #46908. Fixes #53263.

Documents the cause of #50648.

Special notes for your reviewer:

This does not document how a user is expected to find the alertmanagerUid during provisioning. It's possible to hardcode the uid in the Alertmanager data source when provisioning it, then use that UID as the alertmanagerUid, but it's unclear whether that's advisable. See #46908 for a discussion of this problem.

[grafanabot]

Drone build failed: https://drone.grafana.net/grafana/grafana-enterprise/37761

[grafanabot]

This pull request was removed from the 9.2.1 milestone because 9.2.1 is currently being released.

[gguillotte-grafana]

Moving out of draft to request a review. I would prefer to resolve the question about alertmanagerUid best practices before merging this, but wouldn't block on it.

[grafanabot]

Drone build failed: https://drone.grafana.net/grafana/grafana-enterprise/39044

[grafanabot]

This pull request was removed from the 9.2.2 milestone because 9.2.2 is currently being released.

[grafanabot]

This pull request was removed from the 9.2.5 milestone because 9.2.5 is currently being released.

[grafanabot]

This pull request was removed from the 9.2.6 milestone because 9.2.6 is currently being released.

[armandgrillet]

@brendamuir What should we do with that PR? I see that the branch has conflicts with main.