docs: improve callout and voice widget LLM exports

[Devon-White]

Part 2 of #388, stacked on #610.

• hides all 17 VoiceWidget instances and widget-only instructions from .md exports
• preserves the existing provider voice documentation as the machine-readable source
• adds targeted titles where legal, credential, network-security, or availability warnings otherwise lose intent
• extends the local/manual preview assertions across all canonical TTS provider routes; the checker is intentionally not wired into CI

Validation:

• yarn test:scripts
• yarn fern-md-check
• yarn check-llm-exports after the Fern preview deploys

Retarget to main after #610 merges.

[github-actions]

🌿 Preview

https://signalwire-preview-docs-611.docs.buildwithfern.com/docs

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View workflow run

[hey-august]

Looks good!

• All 17 instances are inside balanced blocks, and the only substantive prose hidden with them is duplicated elsewhere on the page, so nothing drops out of the exports.
• The 8 new callout titles name the actual hazard and add no new claims on the compliance pages.
• All 16 manifest routes match the pages' slug: frontmatter, including the non-obvious gcloud and amazon-polly ones.
• yarn test:scripts and yarn fern-md-check pass.

One thing I fixed on the branch (70301b4): google and polly were the only provider pages where the ## Voices section contained nothing but the ignored widget block, so the .md export rendered an empty heading. Each now has a one-line pointing at the provider's voice list.

Note: the new paths array expansion in check-llm-exports.mjs has no unit test, so a malformed manifest entry would silently produce zero checks.