docs: shorten alert text and move the detail to the monitoring page

[cplieger]

Summary

• Shortens every shipped alert rule's text so a notification reads at a glance: summaries of 12 words or fewer, descriptions of 40 words or fewer, and a short file header.
• Moves the longer explanations, citations, version notes and severity reasoning to the Alerting section of docs/monitoring.md, so nothing is lost.
• No rule expression, duration, severity or name changes.

Testing

• A measurement script checks every limit; the YAML parses and markdownlint passes.
• The tests that read the alert files pass.
• A second review checked that no fact was dropped and that every message stays true to its rule.