Add Map Enum Transformation Method

[patricijabrecko]

This pull request adds documentation for a new enum mapping transformation method to the templates/initial_domain_mapping_boilerplate.plain file. The update explains how to configure the mapping of external enum fields to DevRev enums using the new method.

Related issue: https://app.devrev.ai/devrev/works/ISS-210921

[Copilot]

Pull Request Overview

This PR adds documentation for a new enum mapping transformation method to the initial domain mapping boilerplate template. This enhancement allows users to configure the mapping of external enum fields to DevRev enums.

• Documents the Map Enum Transformation Method with configuration details
• Provides JSON structure for the transformation_method_for_set field
• Explains the mapping between external enum values and DevRev enum values

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