docs: update WDK release docs for June releases

Created by: ihsraham

Summary

• Update the changelog for the June 18-19 WDK package releases.
• Document the new signer interface, policy argument snapshot behavior, seed encryption helpers, Spark enableLogging, Tron GasFree activationFee, and Solana keyPair.privateKey nullability.

Release Coverage

• wdk-core v1.0.0-beta.12: https://github.com/tetherto/wdk/releases/tag/v1.0.0-beta.12
• wdk-wallet v1.0.0-beta.11: https://github.com/tetherto/wdk-wallet/releases/tag/v1.0.0-beta.11
• wallet-solana v1.0.0-beta.10: https://github.com/tetherto/wdk-wallet-solana/releases/tag/v1.0.0-beta.10
• wallet-spark v1.0.0-beta.20: https://github.com/tetherto/wdk-wallet-spark/releases/tag/v1.0.0-beta.20
• wallet-tron-gasfree v1.0.0-beta.7: https://github.com/tetherto/wdk-wallet-tron-gasfree/releases/tag/v1.0.0-beta.7
• wdk-utils v1.0.0-beta.7: https://github.com/tetherto/wdk-utils/releases/tag/v1.0.0-beta.7

Validation

• git diff --check
• npm run check:meta
• npm run check:redirects
• LINK_CHECK_EXTERNAL=false npm run check:links
• npm run build