[Serverless] Document impact of using logsDB for security users

[nastasha-solomon]

Partially addresses #5864 by providing Serverless docs.

Previews:

• Detection engine overview | Using logsDB index mode: Added a new section to the end of the page that directs users towards the new page that'd dedicated to explaining how rules and alerts are impacted when logsDB is enabled.
• The impact of usage of logsDB index mode with Elastic Security Serverless

[github-actions]

A documentation preview will be available soon.

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[approksiu]

Minor comment, and the rest looks good to me! Thanks a lot!

[jmikell821]

Just a few slight suggestions, thanks!