docs: use vapi.send add-message instead of nonexistent vapi.addMessage

[chiranjeet-vapi]

Description

• fern/tools/client-side-websdk.mdx tells readers to call vapi.addMessage(...). That method does not exist in @vapi-ai/web (2.7.1, dist/vapi.d.ts), so the snippet throws.
• Replaced every mention with vapi.send({ type: 'add-message', message: { role, content } }), the AddMessageMessage shape the SDK exports, and mentioned triggerResponseEnabled.

Evidence

export interface AddMessageMessage {
    type: 'add-message';
    message: ChatCompletionMessageParam;
    triggerResponseEnabled?: boolean;
}
send(message: VapiClientToServerMessage): void;

Testing Steps

• fern check (pinned 5.151.1) passes locally
• Check the preview deployment for the changed page

[lightsage-app]

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Commit: b75baf4
Status: waiting for staging docs URL