How can I use the Courier plugin to notify a user when an entry is enabled?
In your Courier Blueprint listen to entry.beforeSaveEntry events and use this code for the Trigger Condition:

{% set oldEntry = craft.entries({
    id: entry.id,
    status: null,
}).first() %}

{%- set wasDisabled = oldEntry.status == 'disabled' %}
{%- set isEnabled = entry.status == 'live' %}

{{- wasDisabled and isEnabled ? 'true' : 'false' }}