docs: update confidential ai API docs

[Marvin-Cypher]

Summary

• Add Confidential AI API reference pages for chat completions, models, attestation, signatures, embeddings, and embedding model listing
• Refresh On-demand API model catalog and examples from the RedPill docs update
• Update verification docs for provider-dependent attestation formats and signature text formats
• Fix stale/broken model IDs and examples in streaming, vision, structured output, and tool calling docs

Validation

• node -e "JSON.parse(require('fs').readFileSync('docs.json','utf8')); console.log('docs.json ok')"
• git diff --cached --check before commit
• scanned for RedPill SDK/CLI references and stale model/base URL patterns

Note: This intentionally does not introduce RedPill verifier SDK/CLI docs yet.