docs(saa-js): document turnReady event payload field types (#22)

[MunawarJohar]

Closes #22

Problem

The README listed the turnReady payload shape ({ audioBase64, audioPcm16, durationSec, frames, context }) with no types or
descriptions, and was missing the serverTurnReadyTsMs field entirely.
Consumers had to read the SDK source to know field types.

Fix

• Added serverTurnReadyTsMs to the Events table payload summary.
• Added a turnReady fields table directly below the Events table,
  documenting type and description for every field, verified against
  the emit("turnReady", ...) call site in client.ts's
  handleServerMessage().

Documentation only — no code changes.