docs(fields): Add logging field variant versioning documentation

Description

Add documentation for the logging field variant versioning system introduced in !509 (merged).

This addresses the review feedback requesting a full explanation of the dual-emit and variant emit behavior.

The new v2/doc/LOGGING_FIELD_VARIANTS.md covers:

• Key concepts: schema version, dual-emit target, and variant version definitions
• Storage and implementation: logging_settings JSONB column, jsonb_accessor usage, validation rules, and REST API exposure
• Feature flag: logging_field_variant_versioning WIP flag gating the admin settings
• Emission decision tree: how the schema version and dual-emit target determine which field names appear in log output
• Migration path: the three-step upgrade workflow, version skipping, and cost considerations
• ServiceMapping API (planned): construction, field emission methods, and helpers for consuming services

Related

• Directly related: gitlab#596493 (closed)
• Review comment: !509 (comment 3391477918)
• Epic: gitlab-org/quality#398
• DB migration (JSONB column): gitlab!237869 (merged)
• Backend logic (validations, API, feature flag): gitlab!237907 (closed)