[DOCS] Add docs for moving runtime field definitions to index mapping

[lockewritesdocs]

#69531 introduced the ability to calculate a value for long and double fields at index time. The script parameter on these fields allows users to take a runtime field definition, remove it from the runtime portion of the mappings, and add it to their index mappings to achieve better performance.

If a user moves their runtime definition to the index mappings, that script becomes immutable. To rectify any issues with the script in the index mappings, users would need to create a runtime field with the same name as the indexed field to override the script in the index mappings.

We need to document how users can move their runtime field definitions (and scripts) to their index mapping, and how to rectify any mistakes after indexing.

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