ALert Flow basic explanation. (#1239)

# What this PR does

Updating docs with basic alert flow explanation.

## Which issue(s) this PR fixes

## Checklist

- [ ] Tests updated
- [x] Documentation added
- [ ] `CHANGELOG.md` updated

---------

Co-authored-by: Joey Orlando <joey.orlando@grafana.com>
Co-authored-by: Ildar Iskhakov <Ildar.iskhakov@grafana.com>

docs/sources/get-started/_index.md

@@ -66,6 +66,31 @@ send a demo alert.
 For more information on Grafana OnCall integrations and further configuration guidance, refer to
 [Grafana OnCall integrations]({{< relref "../integrations" >}})
 
+### Learn Alert Flow
+
+All Alerts in OnCall are grouped to Alert Groups ([read more about Grouping ID]({{< relref "../alert-behavior/alert-templates" >}})). Alert Group could have mutually
+exclusive states:
+
+- **Firing:** Once Alert Group is registered, Escalation Policy associated with it is getting started. Escalation policy will work while Alert Group is in this status.
+- **Acknowledged:** Ongoing Escalation Chain will be interrupted. Unacknowledge will move Alert Group to the "Firing" state and will re-launch Escalation Chain.
+- **Silenced:** Similar to "Acknowledged" but designed to be temporary with a timeout. Once time is out, will re-launch Escalation Chain and move Alert Group
+to the "Firing" state.
+- **Resolved:** Similar to "Acknowledged".
+
+Possible transitions:
+
+- Firing -> Acknowledged
+- Firing -> Silenced
+- Firing -> Resolved
+- Silenced -> Firing
+- Silenced -> Acknowledged
+- Silenced -> Resolved
+- Acknowledged -> Firing
+- Acknowledged -> Resolved
+- Resolved -> Firing
+
+Transitions change trigger Escalation Chain launch with a few-seconds delay to avoid unexpected notifications.
+
 ### Configure Escalation Chains
 
 Escalation Chains are customizable automated alert routing steps that enable you to specify who is notified for a certain