Document a tested JSON response fallback for unknown URLs - #534

Refs #383.

Document the existing catch-all handler recipe suggested in that discussion for APIs that return JSON from unknown URLs. Register a persistent fallback after specific persistent handlers, and show a known URL returning its own response and a dynamically generated unknown URL returning JSON with status 404.

The documentation explicitly explains that the fallback handles the request, so it will not be reported as unexpected by check_assertions(). This documents existing functionality; it does not add a custom no-handler response API or change unmatched-request behavior.

The how-to uses literalinclude to keep the example synchronized with an executable test. Validation on Windows / CPython 3.12.10 / pytest 9.1.1:

• Examples suite: 28 passed, 1 existing xfailed. The new example verifies the specific handler, fallback status/body/content type, and successful assertion checking.
• All applicable configured pre-commit hooks pass, including Ruff, Black, isort, and blacken-docs.
• Sphinx HTML build succeeds; the new recipe and example are present. Strict builds report 50 existing warnings on both current upstream and this branch (duplicate section labels and other existing documentation configuration issues). Their normalized warning lists match exactly; this change adds no warnings.
• git diff --check passes. No production source changes.

Prepared with assistance from OpenAI Codex; the agent wrote and executed the example and documentation checks.