Retitle the Fleet data retention tutorials with task-based titles

[vishaangelova]

Summary

The four ILM tutorials under Data streams were titled and navigated as Scenario 1–Scenario 4, which left the left nav showing nothing but the numbers. The numbers don't encode a sequence — the pages are alternatives, and every cross-reference between them treats them as such — so the label was one the reader couldn't decode without going back to the hub. This PR gives each page a verb-first H1 that states the outcome and a short nav title that differentiates at a glance, and drops the content-type prefix from the two tutorial titles in the same section.

No URLs, filenames, or heading anchors change, so no redirects are needed.

Resolves #7852

Changes

• Scenario pages: New H1s and navigation_title values for data-streams-scenario1.md through data-streams-scenario4.md. The nav labels drop the verb because the parent entry already supplies it, and repeating "Apply an ILM policy to" four times would push the distinguishing words to the right of every label. Scenario 3's H1 also now states its outcome (one data stream in one namespace) rather than its setup.
• reference/fleet/data-streams-ilm-tutorial.md: Drops the Tutorials: prefix from the H1, renames the Identify your scenario heading, and rewrites the list so each entry leads with the page's task and follows with the mechanism it uses, which is what makes the options easy to tell apart.
• reference/fleet/data-streams-pipeline-tutorial.md: Drops the Tutorial: prefix from the H1.
• Cross-links: Replaces the eight in-prose Scenario N references with descriptive link text, and normalizes inbound link text for both tutorial pages, which had drifted into four variants across the docset.

Nav before and after

Data streams/                                    Data streams/
└── Tutorials: Customize data retention          └── Customize data retention policies/
    policies/                                        ├── All data streams, all namespaces
    ├── Scenario 1                        →          ├── Specific data streams, all namespaces
    ├── Scenario 2                                   ├── One data stream, one namespace
    ├── Scenario 3                                   └── Custom integrations
    └── Scenario 4

Notes for review

• Based on the 7701-fleet-namespace-ilm-policy branch so it can update data-streams-namespace-ilm.md, which doesn't exist on main yet. GitHub will retarget this to main once Document assigning an ILM policy to an integration namespace #7808 merges.
• Document assigning an ILM policy to an integration namespace #7808 has a pending commit that rewrites the last bullet of the tutorial hub list into a note. Expect a one-line conflict there on rebase; keep the note and the retitled bullets above it.
• Ordinary in-body uses of "scenario" ("In this scenario, you have {{agent}}s collecting…") are deliberately left alone. They read as plain English and are used consistently across these tutorials, including the ingest pipeline one.
• The published URLs still contain scenario1–scenario4. Renaming the files would pull in redirects for every page plus the legacy mapped_pages entries, so that's left out of scope.

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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