docs(discord): fix Server Members Intent + SSRC-mapping drift; add /voice join slash Choice

Salvage of #11350. Kept:
- Code: add an explicit /voice join Choice in the slash UI (runner accepts both 'join' and 'channel' but only 'channel' was in autocomplete).
- Docs: Server Members Intent is conditional (only needed if DISCORD_ALLOWED_USERS contains usernames); SSRC → user_id mapping uses the voice websocket SPEAKING opcode, not the Members intent.

Dropped from the original PR:
- HERMES_DISCORD_VOICE_PACKET_DUMP — this env var doesn't exist on main (it was in a different PR that isn't merged).
- DISCORD_PROXY docs — already documented on current main.
- DISCORD_ALLOW_MENTION_* docs — already on main.
- "barge-in mode" rewrite — current main actually does pause the listener during TTS (VoiceReceiver.pause() at discord.py:192); there is no barge_in_guard/barge_in_rms on main.

Co-authored-by: Michel Belleau <michel.belleau@malaiwah.com>

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

@@ -281,10 +281,10 @@ In the [Developer Portal](https://discord.com/developers/applications) → your
 | Intent | Purpose |
 |--------|---------|
 | **Presence Intent** | Detect user online/offline status |
-| **Server Members Intent** | Map voice SSRC identifiers to Discord user IDs |
+| **Server Members Intent** | Resolve usernames in `DISCORD_ALLOWED_USERS` to numeric IDs (conditional) |
 | **Message Content Intent** | Read text message content in channels |
 
-All three are required for full voice channel functionality. **Server Members Intent** is especially critical — without it, the bot cannot identify who is speaking in the voice channel.
+**Message Content Intent** is required. **Server Members Intent** is only needed if your `DISCORD_ALLOWED_USERS` list uses usernames — if you use numeric user IDs, you can leave it OFF. Voice-channel SSRC → user_id mapping comes from Discord's SPEAKING opcode on the voice websocket and does **not** require the Server Members Intent.
 
 #### 3. Opus Codec