CPS: Document cross-project search support in Maps

[florent-leborgne]

Summary

Adds a new "Search across linked projects" section to the Maps page (maps.md), explaining that:

• Maps supports CPS with an editable project picker scope
• Vector layers and joins pull data from linked projects based on the current CPS scope
• Map panels in dashboards display a "Custom CPS scope" badge when layers use specific project routing

Closes elastic/docs-content-internal#851

Test plan

• Verify the page builds correctly at explore-analyze/visualize/maps
• Confirm applies_to tags render properly (serverless preview / stack unavailable)
• Check cross-links to CPS and dashboard 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]

✅ Vale Linting Results

No issues found on modified lines!

---

The Vale linter checks documentation changes against the Elastic Docs style guide.

To use Vale locally or report issues, refer to Elastic style guide for Vale.