docs: add a Voice Mode page to Chat Features

[silentoplayz]

Summary

Voice Mode had only a one-line bullet on the Chat Features page, plus a permission row, a URL parameter and a keyboard shortcut spread across other pages. This adds a Voice Mode page to Chat Features and links the existing bullet to it.

The page covers:

• starting and ending Voice Mode, and why the button can be missing (more than one model selected, text in the message box, the Allow Call permission)
• how a turn works: recording starts on speech and sends after about two seconds of silence, replies are spoken a sentence at a time as they stream, tap or speak to interrupt, and muting
• camera and screen share, which attach a still frame to each message
• which voice is used, playback speed, the text-to-speech engine, and how Response Splitting changes when speech starts
• that speech is always transcribed on the server, so Voice Mode is unavailable when the admin's speech-to-text engine is Web API
• the two Voice settings under Settings > Interface, the Voice Mode Prompt, the Allow Call permission, and the error messages users see

Related issue or discussion

None. The page was listed as missing in #2 (comment) ("Call User Interface").

Checklist

• I have reviewed the relevant documentation and matched the existing style.
• This PR meets Open WebUI's contribution standards: it is accurate, relevant to users, narrowly scoped, maintainable, and not promotional content, advertising, lead generation, SEO placement, or a request to list a product, service, provider, integration, gateway, tool, or company primarily for visibility.
• I understand that PRs that do not meet these standards may be closed without review and will not be merged. Repeated, low-quality, off-topic, promotional, or intentionally misleading submissions may result in the contributor being blocked from future participation in Open WebUI repositories.

Notes for reviewers

Every behavior on the page was checked against dev at 015dbc8: CallOverlay.svelte (silence detection, sentence playback, interrupt, mute, camera and screen share, voice selection), the Voice mode button in MessageInput.svelte, dispatchCallOverlayAudio in Chat.svelte, the voice prompt in utils/middleware.py, and the setting labels in the en-US translations. It follows the layout of Multi-Model Chats, including the Permissions section.

The page built and rendered with docusaurus start, and every internal link and anchor on it resolves.

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