CPS: Document space-level CPS scope for alerting rules

[florent-leborgne]

Summary

Adds a new "Cross-project search scope for rules" section to the alerting rules page (create-manage-rules.md), explaining that:

• Alerting rules query data across linked projects based on the space-level CPS scope, not a per-rule setting
• The project picker on rule pages is read-only -- to change which projects a rule queries, update the space's CPS configuration

This reflects the actual implementation, which differs from the original issue description (no per-rule CPS scope selector).

Closes elastic/docs-content-internal#688

Test plan

• Verify the page builds correctly at explore-analyze/alerting/alerts/create-manage-rules
• Confirm applies_to tags render properly (serverless preview / stack unavailable)
• Check cross-links to CPS pages resolve correctly

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: Claude (Cursor)

Made with Cursor

[github-actions]

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