Document supervisor call monitoring (listen in, whisper, barge)

[hey-august]

Description

Add a guide for supervisor call monitoring, and correct errors in relevant references and examples.

New

• Supervisor call monitoring guide at /docs/platform/voice/supervisor-call-monitoring.
  Leads with the conference-only constraint, maps listen in / whisper / barge onto
  muted + coach, shows the join in SWML, the REST participant update that
  switches modes mid-call, and an end-to-end Server SDK flow in Python and TypeScript.
  Includes limits, a "Not legal advice" consent warning, and a disambiguation list
  (call whisper, AI barge, ai_sidecar, stream/tap).

Fixes

• coach direction was inverted on all four SDK join-conference pages, the cXML
  <Conference> noun, and the SWML join_conference reference and .tsp. The
  parameter names the participant being coached; the call that sets it is the coach
  (verified against source code).
• SWML coach now says UUID call ID, matching the validator and its own example.
• cXML: prose added to the coaching example, empty Ruby coach SID and single-quoted
  C# string fixed, unverifiable 13240 claim replaced with the actual behavior
  (DialCallStatus=busy).
• SDK update-participant no longer reads as mute/hold only; coaching example added.
• Getting-started no longer promises "live call monitoring" from the RELAY client.
• Cross-links from the voice overview, call-whisper recipe, and affected reference pages.

Not in this PR

• Relay REST conferences API is missing from specs/ entirely (needs .tsp work).
• SWML join_conference_result documented values don't match the engine (joined/failed).

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Code cleanup / refactor

Related Issues

https://github.com/signalwire/cloud-product/issues/20825

Testing

• Added/updated unit tests
• Tested manually
• Tested with live SignalWire credentials (if applicable)

Checklist

• I have read the CONTRIBUTING guidelines
• My code follows the project's style guidelines
• I have added tests for my changes (if applicable)
• I have updated documentation (if applicable)
• All existing tests pass

[github-actions]

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