docs: add LLM equivalents for Dashboard screenshots

[Devon-White]

Summary

• hide Dashboard and related instructional screenshots and their expiring image URLs from Markdown exports
• add page-specific text equivalents for navigation, settings, wizard state, and the two legacy SWML Dashboard screenshots
• share only the nine identical TTS assignment blocks
• add local/manual deployed-preview assertions for every affected route and key workflow; the checker is intentionally not wired into CI

Scope

This is PR 4 in the stacked implementation for #388 and depends on #612. The final audit also found and covered five screenshots stored outside the /assets/images/dashboard/ path: three WireStarter/SWSH captures and two legacy SWML Dashboard captures.

Validation

• yarn test:scripts
• yarn fern-md-check
• git diff --check
• yarn check-llm-exports after the Fern preview deploys
• source audit confirms every Dashboard image reference is inside llms-ignore
• the manual checker rejects presigned image URLs on every tested route

Refs #388

[github-actions]

🌿 Preview

https://signalwire-preview-docs-613.docs.buildwithfern.com/docs

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[hey-august]

Edits 2 snippets that aren't used by anything. Should probably just delete those snippets before merge:

• fern/snippets/dashboard/ui-accordion.mdx was orphaned on 2026-02-23 (7ce1aab "remove old guides"). The six pages that render a UI accordion today all use the different file fern/snippets/common/dashboard/_ui-accordion.mdx — the leading-underscore sibling — which is alive and untouched by #613.
• fern/snippets/common/call-fabric/resource-subscribers.md was orphaned on 2026-02-25 (deee06f "delete CF overview").

[hey-august]

Approving.

• Verified 13 screenshots against their text equivalents — all UI labels and navigation paths match.
• All nine TTS assignment blocks replaced by the shared snippet were byte-identical beforehand.
• Manifest routes match page slugs; every affected page is covered by the presigned-URL check.

I verified that, after the rebase, the content is identical to what I first reviewed.