Update FOCUS converter documentation for 1.2-preview specification

[Copilot]

This PR updates the FOCUS converter documentation to include the newly added fields in the FOCUS 1.2-preview specification, ensuring users have complete guidance for converting Cost Management data to the latest FOCUS format.

Key Changes

New FOCUS 1.2 Fields Added:

• ServiceSubcategory - Added mapping with lookup to Services data file

Column Promotions and Renames:

• Updated InvoiceId mapping notes (promoted from x_InvoiceId in FOCUS 1.0)
• Updated PricingCurrency mapping notes (promoted from x_PricingCurrency in FOCUS 1.0)
• Updated SkuMeter mapping notes (renamed from x_SkuMeterName in FOCUS 1.0)
• Added SkuPriceDetails as new column that will eventually replace x_SkuDetails

Documentation Improvements:

• Added comprehensive "Note for FOCUS 1.2 users" section explaining all changes
• Updated document description to mention FOCUS 1.2-preview support
• Clarified that x_SkuMeterName was renamed (not deprecated) to SkuMeter

The documentation now provides complete coverage for both FOCUS 1.0 and 1.2-preview specifications, focusing only on native FOCUS columns as intended, helping users understand the evolution of the FOCUS standard and how to properly convert their Cost Management data to the latest format.

Fixes #1817.

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