CPS in Kibana: scope selector and availability matrix

[florent-leborgne]

Summary

Adds a dedicated "Managing CPS scope in your project apps" overview page (cross-project-search-manage-scope.md) under explore-analyze/cross-project-search/:

• Scope hierarchy: introduces the three levels of CPS scope control (space default, session scope, query-level override) upfront.
• Scope selector: describes the CPS scope selector in the header, its options (This project, All projects), and session persistence behavior.
• Query-level overrides: documents how to override the scope using SET project_routing or qualified index expressions in ES|QL FROM commands, with a concrete ES|QL example.
• Availability matrix: table showing which apps support the CPS scope selector and query-level overrides, covering 10 apps (Discover, Dashboards, Lens, Maps, Vega, Rules, Dev Tools, ML Data Visualizer, Streams, Agent Builder).
• Related pages: links to CPS overview, project routing, search behavior, admin configuration, ES|QL in Kibana, and the ES|QL CPS reference (cross-link to elasticsearch repo).

Closes elastic/docs-content-internal#850
Closes elastic/docs-content-internal#852
Closes elastic/docs-content-internal#853

Known issues (preview)

• ES|QL editor underlines qualified index names as "unknown index": When using qualified expressions like project_alias:kibana_sample_data_ecommerce in an ES|QL FROM command, the editor flags the index as unknown (red underline), but the query executes successfully and returns correct results. This appears to be a client-side validation gap in the ES|QL editor that doesn't yet account for CPS qualified expressions. Likely to be fixed before GA.
• _origin: qualified expression not working: Using FROM _origin:index_name to target only the origin project does not work in the current preview, despite being documented in the ES|QL CPS reference. This may be a not-yet-implemented feature.

Test plan

• Verify the page builds correctly
• Verify scope selector behavior (session persistence, space default as fallback) against Kibana source
• Verify Rules row (read-only scope) against Kibana source
• Verify qualified expressions work in ES|QL FROM (tested with real linked project)
• Verify SET project_routing works in ES|QL (tested with real linked project)
• Cross-links validated (all resolve to existing pages/anchors on feature branches)
• Content type check: passes overview guidelines (type field, required elements, best practices)

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)

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[florent-leborgne]

Merging this one to the feature branch for now - we have time to make further adjustments if some things are still pending on the product side or require further clarifications;