Add documentation on how to use Alert Topics for multiple PagerDuty services

Currently you can only configure a single PageDuty

```
[pagerduty]
# Configure PagerDuty.
enabled = false
# Your PagerDuty Service Key.
service-key = ""
# The PagerDuty API URL should not need to be changed.
url = "https://events.pagerduty.com/generic/2010-04-15/create_event.json"
# If true the all alerts will be sent to PagerDuty
# without explicitly marking them in the TICKscript.
global = false
```

The ability to allow for having multiple PagerDuty configurations would we great. This is motivated by a user that has a separate PagerDuty account for each team.

Start with the existing [pagerduty] configuration shown in the issue and trace how Alert Topics and PagerDuty services are represented. The issue names no files or tests, so first identify the relevant configuration and alerting entry points; done should include a decided approach for multiple service configurations and documentation showing how teams use it.

Written by the indexing model from the issue text.