docs(community): add Bandwidth telephony serializer

[kshahbw]

Summary

Adds Bandwidth as a community-maintained Telephony Serializer integration.

• Repository: https://github.com/Bandwidth/pipecat-bandwidth
• Maintainer: Bandwidth (@Bandwidth) — official integration from the Bandwidth team
• Demo video (~100s): https://drive.google.com/file/d/1AjMwF49i-e3FHu_d5rIP91vtK12IoSFL/view?usp=sharing
• Bandwidth Developer Documentation: https://dev.bandwidth.com/docs/voice/integrations/pipecat

The repo includes:

• BandwidthFrameSerializer — handles Bandwidth Programmable Voice's bidirectional WebSocket protocol
• Inbound μ-law decoding and outbound μ-law or linear PCM at 8/16/24 kHz (PCM at 24 kHz noticeably improves TTS quality)
• Interruption handling via Bandwidth's clear event
• Auto hang-up via the Bandwidth Voice API using OAuth 2.0 client_credentials
• 20 unit tests (passing) + a single-file foundational FastAPI example at examples/bandwidth-chatbot/

Tested with Pipecat v1.1.0 and validated end-to-end on a real Bandwidth Programmable Voice call.

Test plan

• Demo video shows: bot answering an inbound call, multi-turn conversation, mid-response interruption, clean hang-up
• Audio path: inbound μ-law → STT (Deepgram in the example) → LLM → TTS → outbound μ-law over the same WebSocket
• OAuth-based hang-up against the Voice API (REST)

Note: the linked repo is being made public alongside this listing. If anything 404s, that's the timing window — happy to keep this in draft until everything is reachable.

[markbackman]

Video looks great, @kshahbw. Thanks for shooting it. Let me know once this is ready for review.