docs: add LLM equivalents for Server SDK diagrams

[Devon-White]

Stacks on #611. Covers all 46 raster diagram references across 30 Server SDK guide pages: human Frames are excluded from Markdown exports, 38 diagrams receive Mermaid equivalents, five receive structured-Markdown equivalents, and three Dashboard captures rely on complete adjacent numbered instructions. Adds a diagram-specific authoring rule and local/manual preview acceptance cases. The export checker is intentionally not wired into CI.

Validation:

• manually compared all 46 source assets with their LLM-view content
• yarn test:scripts
• yarn fern-md-check (all 2,824 MDX files valid)
• git diff --check
• yarn check-llm-exports after the Fern preview deploys

Part of #388.

[github-actions]

🌿 Preview

https://signalwire-preview-docs-612.docs.buildwithfern.com/docs

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View workflow run

[hey-august]

Looks good!

• All 46 diagram references are inside <llms-ignore> (38 Mermaid diagrams, 5 structured-Markdown equivalents, 3 covered by adjacent steps)
• Image equivalents look good
• All 38 Mermaid fences have correct syntax.
• Manifest routes match page slugs.

I fixed a casing error in the call-transfer table and the manifest. It said final=true when it was supposed to be final=True.

The merge-base changed after approval.

[hey-august]

Approving again.

Post-rebase content verified identical to what was reviewed. The mdx-diagrams.md merge with main's themed-SVG rule resolves cleanly.