Improve format of Signal messages

[doobry-systemli]

Currently they look like this:

❗Alert InstanceDown is firing

Labels:

• alertgroup: ansible managed alert rules
• alertname: InstanceDown
• instance: host.example.org:9100
• job: node
• severity: critical

Annotations:

• description: host.example.org:9100 has been down for more than 10 minutes.
• summary: Instance down

http://metrics/vmalert/alert?group_id=***&alert_id=***

My idea would be to reformat them that way (Description first, no further Annotations and no URL):

❗Alert InstanceDown is firing

host.example.org:9100 has been down for more than 10 minutes.

Labels:

• alertgroup: ansible managed alert rules
• alertname: InstanceDown
• instance: host.example.org:9100
• job: node
• severity: critical