Update server event page with latest events

[sahilsuman933]

Description

• Updated fern/server-url/events.mdx to reflect all current server events and their structures as defined in the OpenAPI schema.
• Replaced outdated "function-call" terminology and examples with "tool-calls".
• Added documentation for newly supported server event types, including conversation-update, transcript, speech-update, model-output, transfer-destination-request, transfer-update, user-interrupted, language-change-detected, phone-call-control, knowledge-base-request, voice-input, voice-request, call.endpointing.request, chat.*, and session.*.
• Clarified which events require a response from the server and provided updated request/response examples.

Testing Steps

• Run the app locally using fern docs dev or navigate to preview deployment
• Ensure that the changed pages and code snippets work

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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