Describe Default Receiver in the Alertmanager Configuration

[network-charles]

This update describes the required default receiver on the configuration page.
Developers may not know that not specifying a default receiver may throw an error message saying:

err="root route must specify a default receiver"

Though the default receiver attribute was specified in the example below, it wasn't clear if it was simply an example or a requirement.

Example

# The root route with all parameters, which are inherited by the child
# routes if they are not overwritten.
route:
  receiver: 'default-receiver'
  group_wait: 30s