docs(incidentio): add incident.io permission requirement to install alert (#18263)

## What changed and why

Added incident.io Admin or Owner permission requirement to the install
alert on the incident.io integration page. Previously the alert only
mentioned Sentry-side permissions, leaving the incident.io-side
requirement undocumented.

Follows the same pattern used by other integrations (e.g. GitHub:
"Sentry owner, manager, or admin permissions, and GitHub owner
permissions are required to install this integration.").

## Verification

Content-only change. The new alert text:
> Sentry owner, manager, or admin permissions, and incident.io Admin or
Owner permissions are required to install this integration.

## What's not verified

incident.io's exact permission tier names — confirmed as Admin or Owner
based on the customer report in #discuss-docs. Worth a quick sanity
check against incident.io's docs if desired.

---
Action taken on behalf of Michael Chai.

---
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Co-authored-by: sentry-junior[bot] <264270552+sentry-junior[bot]@users.noreply.github.com>
Co-authored-by: URCU7R7U2 <rahul.chhabria@sentry.io>

Author: sentry-junior[bot]

docs/integrations/issue-tracking/incidentio/index.mdx

@@ -15,7 +15,7 @@ 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.
+Sentry owner, manager, or admin permissions, and incident.io Admin or Owner permissions are required to install this integration.
 incident.io **won't** work with self-hosted Sentry.
 
 </Alert>