docs: add September 24 billing release notes

[yechang15]

Summary

• Add the September 24 Updated Billing release notes with Model and Carrier rates, the 30-second domestic outbound cost comparison, and the Billing guide link.
• Preserve preparation-usage caveats, international carrier billing rules, and the fully waived experimental Task Success Fee.
• Restore date-only changelog headings and move non-date topics to bold body text, keeping historical release content intact.
• Add regression coverage for release dates, pricing details, and the Billing link.

Validation

• Full pnpm run validate passed, including all 32 Playwright tests.
• git diff --check passed.

Release

No release needed: documentation and tests only; no package version bump or changeset required.