Add Kotlin logo and reorder SDKs by market-data popularity

[MarketDataApp]

Summary

• Kotlin logo on SDK index — the Java SDK card now shows both the Java and Kotlin logos (mirroring the JS/TS dual-logo treatment) and is titled "Java / Kotlin SDK", reflecting the SDK's first-class Kotlin interop
• SDK ordering by market-data popularity — both the picker cards (sdk/index.mdx) and the autogenerated sidebar (sidebar_position) now order: Postman → Python → JavaScript/TypeScript → Java/Kotlin → Go → PHP. Also fixes a prior sidebar_position tie between JS and PHP (both were 4).

Test plan

• Verify the SDK index picker shows Java + Kotlin logos side by side and the new order at www-staging.marketdata.app/docs/sdk/
• Verify the sidebar reflects the same order
• CI checks green on staging