docs: clarify transcription example setup

[AdamMomen]

Summary

Updates the transcription examples README to cover both available examples:

• transcriber.py for single-participant transcription
• multi-user-transcriber.py for transcribing multiple remote participants

The README now includes:

• the STT setup used by each example
• uv run commands for both examples
• the expected log output shape for the multi-user transcriber
• a short troubleshooting checklist for common setup issues

This also makes two small updates to multi-user-transcriber.py:

• avoid raising KeyError if a participant disconnects before a session is stored
• log the room name when the multi-user transcriber starts

Testing

UV_CACHE_DIR=.uv-cache uv tool run ruff format examples/other/transcription/multi-user-transcriber.py
UV_CACHE_DIR=.uv-cache uv tool run ruff check examples/other/transcription/multi-user-transcriber.py
python3 -m py_compile examples/other/transcription/multi-user-transcriber.py
git diff --check

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[devin-ai-integration]

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