[Observability][Data Set Quality] Create degraded docs rule page

[mdbirnstiehl]

This PR closes #1910 and adds documentation on the degraded docs rule.

I'll create a separate PR for 8.19.

[github-actions]

🔍 Preview links for changed docs

• solutions/observability/incident-management/create-a-degraded-docs-rule.md
• solutions/observability/incident-management/create-manage-rules.md

[florent-leborgne]

LGTM from docs perspective

[yngrdyn]

LGTM, are we planning to have an screenshot of how the rule form looks like, like in other rule types?

[mdbirnstiehl]

LGTM, are we planning to have an screenshot of how the rule form looks like, like in other rule types?

Ah yeah, I can add one real quick. Thanks!