[8.17] [8.17-8.18] More changes to the LogsDB page (backport #6639)

[mergify]

Description

Contributes to #6526, which requests changes to the note about logsdb and additional information to help users who have enabled logsdb index mode or are considering it. The corresponding 9.0 and Serverless docs are at: elastic/docs-content#878

Preview

Using logsdb index mode with Elastic Security: Made the following changes:

• Modified the second sentence in the note as per the request in the doc issue. Now it says: Logsdb index mode is fully supported, and is recommended for all Elastic Security deployments. Users with existing Elastic Security deployments are advised to fully understand and accept the documented changes...
• Added a new section that briefly explains how logsdb index mode might affect CPU and storage usage and links users to an Elastic blog post with recent benchmarks.
• Expanded the "Runtime fields" section by adding guidance for handling runtime fields with scripts that reference the params._source field.

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This is an automatic backport of pull request #6639 done by [Mergify](https://mergify.com).

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