Documentation: Adding a new application to oncall for iris integration.

It's not immediately obvious that for Oncall and Iris integration to work for user import, the database has to be updated with an application entry in the application table. This would be helpful for new users.

In our case, we used the following insert statement:
`INSERT INTO application (`name`,`key`) VALUES ("myapp","mykey");`

and in iris configuration, we added:

```
oncall-app: myapp # application for oncall auth
oncall-key: mykey # API key for oncall auth
```

Find the documentation covering Oncall and Iris integration and review how user import setup is currently explained. Document that an application row must be added to the application table and that the matching oncall-app and oncall-key values must be configured in Iris. Done means a new user can follow the documented setup without discovering this database step separately.

Written by the indexing model from the issue text.