docs: reword `How to sync alert rules from Git` doc so it's clear that the same instructions apply for syncing dashboards

[sinapah]

Please describe the question or issue you're facing with "How to sync alert rules from a Git repository - Observability documentation".
As it's written, the How to sync alert rules from Git doc leaves reader a bit confused since it only subtly mentions how they can sync dashboards. We should reword the title and some parts of the doc so that users know that they can sync Prom and Loki rules as well as dashboards using COS Config.

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Reported from: https://documentation.ubuntu.com/observability/latest/how-to/configure-and-tune/sync-alert-rules-from-git/