Improve Logsdb docs including default values

[salvatore-campagna]

This PR adds detailed documentation for logsdb mode, covering several key aspects of its default behavior and configuration options.

It includes:

• default settings for index sorting (index.sort.field, index.sort.order, etc.).
• usage of synthetic _source by default.
• information about specialized codecs and how users can override them.
• default behavior for ignore_malformed and ignore_above settings, including precedence rules.
• a section on the _ignored_source field and how to retrieve ignored field values.
• explanation of how fields without doc_values are handled and what we do if they are missing.
• guidelines for opting out of LogsDB for logs data streams.

[github-actions]

Documentation preview:

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Pinging @elastic/es-docs (Team:Docs)

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Pinging @elastic/es-storage-engine (Team:StorageEngine)