Document voice input feature for Copilot CLI

[pathum25]

Added explanation of voice input functionality for Copilot CLI, including details on local transcription and text insertion.

Why:

Closes:

What's being changed (if available, include any code snippets, screenshots, or gifs):

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• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

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