docs: add Polymarket TypeScript initialization

[Doomguy-BFG9000]

Summary

• add a TypeScript initialization example for Polymarket
• use the SDK's actual privateKey, proxyAddress, and signatureType options
• match the neighboring setup-guide structure

Verification

• checked constructor and export names against the TypeScript SDK source
• validated the extracted snippet syntax
• git diff --check

Fixes #2198