docs: fix 3 inaccuracies found during code-vs-docs audit

- voice-mode.md: Discord sends native voice bubbles (OGG/Opus flags=8192),
  not MP3 file attachments. Falls back to file only if voice API fails.
- discord.md: Bot requires @mention by default in server channels
  (DISCORD_REQUIRE_MENTION=true). Previous text incorrectly said no
  mention needed.
- index.md: Fix broken ASCII architecture diagram alignment after
  adding Web adapter box.

website/docs/user-guide/features/voice-mode.md

@@ -213,7 +213,7 @@ Voice mode setting is persisted across gateway restarts.
 | Platform | Format | Notes |
 |----------|--------|-------|
 | **Telegram** | Voice bubble (Opus/OGG) | Plays inline in chat. ffmpeg converts MP3 → Opus if needed |
-| **Discord** | Audio file attachment (MP3) | Sent alongside text response |
+| **Discord** | Native voice bubble (Opus/OGG) | Plays inline like a user voice message. Falls back to file attachment if voice bubble API fails |
 
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