Add a new section for transitioning indices to data streams

[yetanothertw]

The goal of this PR is to include the content from the Migrate from Indices to DataStreams knowledgebase article into our ILM-related documentation.

Initially, the suggestion was to include this new content as a new section on the Manage existing indices page. Upon further review of existing content, the Tutorial: Automate rollover seems like a more appropriate home for this content as it already includes two other equivalent use cases:

• Manage time series data with data streams
• Manage time series data without data streams

The reason I think these use cases are equivalent is because they're trying to use ILM policies to migrate from periodic indices to a more automated way to manage rollover and replace the need to schedule or script index creation (one option migrates from indices to data streams and the other one migrates to using aliases in order to manage their backing indices). The new content adds this use case that's equivalent in scope:

• Manage general content with data streams.

Fixes #1571

[github-actions]

🔍 Preview links for changed docs

• manage-data/lifecycle/index-lifecycle-management/manage-existing-indices.md
• manage-data/lifecycle/index-lifecycle-management/tutorial-automate-rollover.md

[yetanothertw]

I'm also wondering whether this tutorial is related to the goal we're trying to achieve in the Migrate from Indices to DataStreams knowledgebase article.
Is the overall goal to automate rollover for static data indexes with data streams? Is that why we'd be migrating from indices to data streams?