Add claim evaluation protocol article

[ig-shaun]

Summary

Adds a concept and architecture article for claim evaluation on IXO and wires it into site navigation.

What changed

• New page articles/claim-evaluation-protocol.mdx: Describes how claim evaluation fits together—Claims, evidence, rubrics, Agentic Oracles, and Universal Decision and Impact Determination (UDID) records—and what accountable outputs look like at a determination point. The page positions itself as conceptual documentation and points readers to the canonical Claims management, Agent evaluations, Developer workflows, and Product and SDK map for implementation detail.
• docs.json: Adds the article path so it appears in the Mintlify navigation.

Validation

• Confirmed GitHub CLI authentication and repository default branch (main).
• Reviewed frontmatter and nav entry for consistency with the rest of the docs repo.