Document assigning an ILM policy to an integration namespace

[vishaangelova]

Summary

This PR documents the Fleet ILM policy picker, which lets you assign an ILM policy to all of an integration's data streams in one namespace from Data retention settings in the integration policy editor, instead of editing component templates by hand. The setting belongs to an (integration, namespace) pair rather than to a single integration policy, and requires namespace index templates to be enabled for that namespace. It targets Stack 9.5+ and is not available on Serverless.

Implementation PR: elastic/kibana#275197

Resolves #7701

Changes

• reference/fleet/data-streams-namespace-ilm.md (new): How-to for assigning, changing, and clearing the ILM policy in the UI and with the API, how Fleet applies it through the managed <type>-<dataset>@namespace.<namespace> component template, when changes take effect, and the feature's limitations.
• reference/fleet/data-streams-namespace-custom.md: Completes the follow-up deferred from Document namespace index templates and overlap resolution #7705 — documents the managed ILM component template in the composed_of chain and notes that opting a namespace out also clears its ILM assignment.
• reference/fleet/data-streams-scenario3.md: Reworks the intro so the scenario's scope is stated up front, and points to the new page for the all-data-streams case. Scenario 3 remains the path for a single data stream in a namespace, which the picker doesn't cover.
• Pointers and cross-links: Adds references to the new page from the ILM tutorial landing page and the Fleet data streams ILM section, and wires the new page into toc.yml.

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: Cursor / Claude Opus 5

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Elastic Docs Style Checker (Vale)

Summary: 3 suggestions found

💡 Suggestions (3): Optional style improvements. Apply when helpful.

File	Line	Rule	Message
reference/fleet/data-streams-scenario3.md	15	Elastic.Clone	Use cloning only when referring to cloning a GitHub repository or creating a copy that is linked to the original. Often confused with 'copy' and 'duplicate'.
reference/fleet/data-streams-scenario3.md	29	Elastic.Clone	Use cloned only when referring to cloning a GitHub repository or creating a copy that is linked to the original. Often confused with 'copy' and 'duplicate'.
reference/fleet/data-streams-scenario3.md	29	Elastic.Clone	Use cloned only when referring to cloning a GitHub repository or creating a copy that is linked to the original. Often confused with 'copy' and 'duplicate'.

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🔍 Preview links for changed docs

• reference/fleet/data-streams-ilm-tutorial.md
• reference/fleet/data-streams-namespace-custom.md
• reference/fleet/data-streams-namespace-ilm.md
• reference/fleet/data-streams-scenario3.md
• reference/fleet/data-streams.md

[jillguyonnet]

LGTM! 🎉

[mdbirnstiehl]

Overall looks good! I had a few questions/comments for your consideration.