Data lifecycle docs: Customize built-in policies tutorial

[kilfoyle]

Feedback related to Customize built-in policies:

• Kubernetes logs are massive so we want a dedicated ILM policy for that, apart from the main one. Need to clone the existing policy, etc. The relevant tutorial is Customize built-in policies so maybe we need to expand on that.

• What is logs@custom that is mentioned in Customize built-in policies?

[kilfoyle]

I've opened #3511 to:

• better explain @custom component templates
• guide users toward the more detailed instructions for working with them in the Fleet docs.

The @custom templates are complicated because not all managed index templates have them, and some have multiple. Also, ILM policies like logs are marked in the UI as deprecated, in favour of logs@lifecycle, but the former still seems to be used by default, at least when I test using the System integration. So, I've limited the discussion to @custom component templates and their parent managed index templates.

As for the "massive Kubernetes logs" scenario, I think the steps to customize the policy are covered nicely in the existing tutorial. I've added a sentence at the top of that page to give this use case as a specific example of why/when we'd want to use a custom ILM policy.

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