[DOCS] Enhance the _source / index.mapping.source docs

[lucabelluccini]

Description

Enhance the _source / index.mapping.source docs to tell what are the available options and how they play with Logsdb and Synthetic _source

Explain how to disable Synthetic _source on specific indices.

Untangle the complexity of the following setting (and which one overrides what on which version)

• mappings._source.mode (synthetic | stored)
• settings.index.mode (time_series | logsdb | standard)
• index.mapping.source.mode (SYNTHETIC | STORED)

Related to #118596 and #116689

[elasticsearchmachine]

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

[lucabelluccini]

FYI @martijnvg

[martijnvg]

mappings._source.mode (synthetic | stored)

We should no longer mention this mapping attribute. I think starting from 8.17.0, this mapping attribute is also no longer documented.

[elasticsearchmachine]

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