chore(incidentio): Update docs with alert stuff (#13882)

## DESCRIBE YOUR PR

Update docs for incident.io. Not sure we should be maintaining all of
these but if they currently exist, it's worth mentioning all the
features..

<img width="664" alt="image"
src="https://github.com/user-attachments/assets/7f922b19-1f52-4a8a-9a13-fbb2aa37515a"
/>


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## SLA

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Thanks in advance for your help!

## PRE-MERGE CHECKLIST

*Make sure you've checked the following before merging your changes:*

- [ ] Checked Vercel preview for correctness, including links
- [ ] PR was reviewed and approved by any necessary SMEs (subject matter
experts)
- [ ] PR was reviewed and approved by a member of the [Sentry docs
team](https://github.com/orgs/getsentry/teams/docs)

Author: leeandher

docs/organization/integrations/issue-tracking/incidentio/img/incidentio-alert-rule.png

@@ -13,7 +13,6 @@ This integration is maintained and supported by the company that created it. For
 <Alert>
 
 Sentry owner, manager, or admin permissions are required to install this integration.
-
 incident.io **won't** work with self-hosted Sentry.
 
 </Alert>
@@ -40,7 +39,7 @@ The Sentry issue is now attached to the incident:
 
 ![Incident with Sentry issue attached](./img/incidentio-attach-post.png)
 
-### In sentry.io
+### In Sentry
 
 To attach a Sentry issue to an incident in [sentry.io](https://sentry.io), navigate to a specific Sentry issue and find the "Linked Issues" section on the right panel. Click "Link incident.io issue" and select your active incident from the dropdown:
 
@@ -51,3 +50,17 @@ To attach a Sentry issue to an incident in [sentry.io](https://sentry.io), navig
 The incident's Slack channel will be updated with attached Sentry issue comments, edited comments, and when the Sentry issue is resolved:
 
 ![Synced comments from Sentry on an incident in Slack](./img/incidentio-issue-sync.png)
+
+
+## Alerting
+
+Once installed, you can also set Sentry to be an alert source for incidents.
+
+![incident.io alert setup](./img/incidentio-alert-setup.png)
+
+Afterward, within Sentry you can connect it to that newly created source in an alert rule.
+
+![incident.io alert rule](./img/incidentio-alert-rule.png)
+
+
+When the alert fires, it'll create an incident on incident.io, with all the downstream affects you set up on that side.