docs: gate cross-project search space panel by pricing tier (#7635)

[florent-leborgne]

Summary

This PR addresses #7635 with the following changes:

• deploy-manage/cross-project-search-config/cps-config-access-and-scope.md: notes that the Cross-project search panel in space settings only appears when the project is on a CPS-eligible pricing tier, cross-linking to the existing tier requirements instead of duplicating them, and documents the downgrade escape hatch (the panel stays visible on a space with a non-default scope until you reset it to All projects).
• deploy-manage/cross-project-search-config.md: adds a one-line reciprocal cross-reference from "Projects available for linking" back to the space-settings panel behavior, since the same tier requirement now gates both.

Resolves

Closes #7635

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AI-generated draft — created with Claude Sonnet 5.
Review all generated content for factual accuracy before merging.

Generative AI disclosure

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

• Yes
• No

Tool(s) and model(s) used: Claude Code, Claude Sonnet 5.

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🔍 Preview links for changed docs

• deploy-manage/cross-project-search-config.md
• deploy-manage/cross-project-search-config/cps-config-access-and-scope.md

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