Add AlertManagementAlerts to OpenAPI documentation

What does this MR do and why?

This MR adds API::AlertManagementAlerts to OpenAPI V2.

It also adjusted some example strings in the REST API documentation to match OpenAPI examples.

From the commit message:

* Add response codes for API endpoints
* Add field documentation for entities

This is needed to show examples nicely in Swagger UI. See screenshots 👇

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/376862.

Screenshots or screen recordings

1. Run bundle rake gitlab:openapi:generate
2. Go to https://editor-next.swagger.io/
3. Import doc/api/openapi/openapi_v2.yaml via File > Import File
4. Wait for it ⏳
5. Search for alert_management endpoints
6. Expand endpoints

https://editor-next.swagger.io/

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.