Add "Route and control calls" guide

[hey-august]

Description

Adds a tutorial at /docs/platform/voice/route-and-control-calls and points the Voice overview's get-started card at it.

The dashboard's new quickstart card "Route and Control Calls" (cloud-product #20653, live since 2026-09-09) currently lands on the goto/execute/transfer disambiguation page, which compares three jump verbs and never answers a call. This guide is the intended target: the reader writes a flow that answers and speaks, calls it with Click-to-Test, adds a speech prompt with switch and transfer into three sections, calls it three times, and reads the timeline under Logs > Voice.

It follows the AI quickstart's shape, with Server SDK and SWML tabs producing the same document. It is the first non-AI Server SDK guide and a template for the planned SWML/Server SDK guide unification.

Verified

• Python example run against the SDK checkout: the document validates against the schema and serve() returns it with Basic Auth.
• Dashboard behavior (Click-to-Test button, alias-address requirement, Logs > Voice timeline contents) checked against prime-rails source.
• yarn fern-md-check passes. yarn fern-check reports only the FERN_TOKEN redirects check.
  Not verified: a live call through the three speech cases. Someone with a Space should place them before merge.

Follow-ups

• Repoint the dashboard quickstart card in cloud-product #20653 to /docs/platform/voice/route-and-control-calls once this merges.
• File the SDK issue on SWMLBuilder being main-only and cond being unemittable (draft in the PR thread). The guide uses SWMLService and switch to work around both.
• The Server SDK "Relay Client" guide still positions Relay as the path for IVR and call routing. That needs a "when to use which" note as part of the unification.
• No changelog entry: documentation work only.

Type of Change

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

Related Issues

#681

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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