alertmanager-irc-relay: Reference the diagram

To generate the svg out of the mermaid source code:

$ docker run --rm -u `id -u`:`id -g` -v $(pwd):/data \
  minlag/mermaid-cli -i alertmanager-irc-relay.mmd

Refs. swh/infra/sysadm-environment#4525

added 18 commits

• a59bee4b...63aca89a - 17 commits from branch swh/devel:master
• 81ee9bdc - alertmanager-irc-relay: Reference the diagram

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merged