Document household API calculate request and response examples

[anth-volk]

Context

The household API docs should include a clearer calculate endpoint reference that explains accepted request keys, required/default behavior, and representative response bodies.

Scope

• Add a calculate endpoint reference section to the country API docs pages.
• Show focused request examples for version, household, policy, enable_ai_explainer, and household.axes.
• Show selectable response examples for success, deprecated-variable warnings, validation failures, auth failures, unsupported countries, and calculation failures.
• Keep examples concise by avoiding full household payload expansion where a placeholder is clearer.

Review note

This documentation exposes the enable_ai_explainer request key and references the AI explainer flow. That endpoint is not used by any customer and is not fully implemented, so reviewers should explicitly decide whether documenting it is acceptable in this PR.