Improve the structure and scannability of the ILM-to-data-stream-lifecycle migration tutorial #9105

Problem

The Migrating ILM-managed data streams to data stream lifecycle tutorial would benefit from a structural refresh similar to the work in #8911.

Source: manage-data/lifecycle/data-stream/tutorial-migrate-ilm-managed-data-stream-to-data-stream-lifecycle.md.

The opening combines an introduction, migration behavior, a prominent Streams admonition, and a two-step summary before readers reach the procedure. The admonition interrupts the explanation and makes the opening harder to scan.

The setup and migration sections then alternate instructions, commands, long responses, and explanations under broad headings. Readers have few landmarks for locating their current step or returning to a verification step.

Suggested changes

• Tighten the opening around the task and expected outcome. Preserve the important distinction that existing backing indices remain managed by ILM while newly created backing indices use data stream lifecycle.
• Move the Streams alternative out of the middle of the introduction, or replace the large admonition with a short contextual link.
• Add a clear prerequisites section.
• Break setup into identifiable tasks: create the ILM policy, create the index template, and create sample data.
• Add subheadings or a stepper for migration: update the index template; configure the existing data stream; verify the next backing index's lifecycle; roll over the data stream; verify the migration.
• Keep expected results and their explanations next to the corresponding action. Consider collapsing lengthy example responses where that improves scanning without hiding essential checkpoints.

Acceptance criteria

• Readers can identify the purpose, requirements, and first action without navigating around unrelated promotional content.
• Setup, migration, verification, and switching back to ILM have clear landmarks.
• Technical behavior, version/deployment scope, existing inbound anchors, and runnable examples are preserved or explicitly corrected as part of the refresh.
• The revised page is checked against the tutorial content-type guidance and inspected in a rendered preview.

This is existing page-level debt noticed during review of #8911, whose change to this page is only a link update. Track it separately so it does not expand or block that PR.