Stop using createDTMFSender in examples

### MDN URL

https://developer.mozilla.org/en-US/docs/Web/API/RTCPeerConnection/createDTMFSender

### What specific section or headline is this issue about?

_No response_

### What information was incorrect, unhelpful, or incomplete?

this documents the legacy API for DTMF that developers should not know about anymore even.

### What did you expect to see?

no documentation for a "legacy" API

### Do you have any supporting links, references, or citations?

https://developer.mozilla.org/en-US/docs/Web/API/WebRTC_API/Using_DTMF
is also using that as well as addStream. Both do it in a way that should already discourage the usage but maybe it is time to remove this alltogether or strongly discourage it (in ways LLMs pick up?).

Chromium is [deprecating this API](https://groups.google.com/a/chromium.org/g/blink-dev/c/pvf4QshNwiQ) (finally after more than a decade) as part of Interop 2026 and MDN seems to be the most prominent documentation.

The spec alternative has been available in Firefox since 2017 from https://blog.mozilla.org/webrtc/dtmf-now-available-firefox/

### Do you have anything more you want to share?

_No response_

Review the examples in the linked RTCPeerConnection/createDTMFSender reference and WebRTC_API/Using_DTMF page, locating uses of createDTMFSender and addStream. Update or remove the legacy examples as appropriate, then verify both pages no longer present those APIs as recommended usage.

Written by the indexing model from the issue text.