Add columnar index mode overview

[lcawl]

Summary

Relates to elastic/roadmap#290

This PR adds an introductory overview of the new columnar index mode (and small links and comments in the related pages in the "Manage data" section of the documentation).

NOTE: Unlike the existing logsdb page (https://www.elastic.co/docs/manage-data/data-store/data-streams/logs-data-stream), this overview does not include details about how to enable the index mode. Those (and other advanced) details are underway in elastic/elasticsearch#151739 If those steps ought to be moved, that can occur as needed.

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Elastic Docs Style Checker (Vale)

Summary: 3 suggestions found

💡 Suggestions (3): Optional style improvements. Apply when helpful.

File	Line	Rule	Message
manage-data/data-store/columnar.md	18	Elastic.Semicolons	Use semicolons judiciously.
manage-data/data-store/columnar.md	60	Elastic.WordChoice	Consider using 'can, might' instead of 'may', unless the term is in the UI.
manage-data/data-store/columnar.md	65	Elastic.Semicolons	Use semicolons judiciously.

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🔍 Preview links for changed docs

• manage-data/data-store/columnar.md
• manage-data/data-store/data-streams/logs-data-stream.md
• manage-data/data-store/index-basics.md

[yetanothertw]

A few thoughts on this:

• a couple of small suggestions for this PR

• and I think we need a follow-up task to add more info into this datastore section about other new index modes (vectordb_document and the lookup mode). I'll create a follow-up item for this (probably will be an admin/dev docs colab).

[yannis-roussos]

Thank you @lcawl, looks great! I have added a few small comments for your consideration.

Once we are done with the release and we have some more spare time to plan, we should think about reorganizing where we talk about the various index modes. Even where LogsDB and TSDB reside today is a little bit weird and with Columnar and Vector also available, we have an opportunity to revisit this

[yetanothertw]

Just fiy, I've opened #7617

Once we are done with the release and we have some more spare time to plan, we should think about reorganizing where we talk about the various index modes. Even where LogsDB and TSDB reside today is a little bit weird and with Columnar and Vector also available, we have an opportunity to revisit this

@yannis-roussos we should be able to track that in this issue ^