Add documentation about configuring logs retention

[gbamparop]

There is some documentation about using ILM policies to configure data retention. A get started guide can be created for Observability users explaining how they can easily configure retention through ILM, while reusing and improving the current docs.

• Tutorial: Customize built-in ILM policies
• Tutorial: Automate rollover with ILM
• ILM: Manage the index lifecycle
• Configure a lifecycle policy

[gbamparop]

I think this is still relevant, regardless of the Streams release as retention can't be set from that UI for classic streams.

Once wired streams are released we could have another guide or update the docs that will be created as part of this issue to mention the other flow too.

cc @LucaWintergerst

[mdbirnstiehl]

On hold until 9.1 to account for new features added here PR 127858.

[LucaWintergerst]

the other PR is in an we have added the functionality to Streams too