docs: Alertmanager reference shows use of {{.recipient}} this isn't possible

The Alertmanager implementation does not extract and pass the recipient for a notification subscription.

Reference: https://github.com/argoproj/notifications-engine/blob/master/docs/services/alertmanager.md?plain=1#L149

The pagerduty service [Send()](https://github.com/argoproj/notifications-engine/blob/master/pkg/services/pagerdutyv2.go#L118) function uses dest.Recipient

In the alertmanager server [Send()](https://github.com/argoproj/notifications-engine/blob/master/docs/services/alertmanager.md?plain=1#L149) function there is not use of dest.Recipient (or in fact dest)

Start with docs/services/alertmanager.md at the referenced line and compare its example with the Alertmanager Send implementation. Review pkg/services/pagerdutyv2.go and its Send function to understand the dest.Recipient usage contrast. Done when the Alertmanager reference no longer claims that an unavailable recipient value can be used.

Written by the indexing model from the issue text.