docs: show field precedence with executable Quick Start examples

[Gonghan-Princess]

The Quick Start describes defaults and static fields without showing their JSON output or explaining which source wins when keys overlap. Add visible outputs and an example showing how defaults, format fields, static fields, dictionary messages, and record extras combine in the current implementation.

Enable INFO logging in the shared setup, correct the defaults argument name, and add tests that execute the actual Markdown examples in fresh subprocesses and compare their output with the displayed JSON. Add an Unreleased changelog entry.

Refs #36. This addresses the examples and field-precedence portion of the issue.

Validation:

• Full pytest suite: 221 passed, including three Markdown example cases.
• Black, mypy, and validate-pyproject passed. Pylint passed at 10.00/10 on an LF export (the Windows checkout produces CRLF-only warnings).
• MkDocs build passed with mkdocstrings-python 1.19.0. The latest 2.0.8 handler rejects the existing import configuration; no dependency or configuration changes are included here.
• Strict MkDocs builds on this change and the unchanged base report the same seven existing source-docstring annotation warnings. Rendered Quick Start output was checked.

AI assistance: This contribution was prepared with OpenAI Codex, including drafting, implementation, and local verification.

[nhairs]

Thanks @Gonghan-Princess - overall this looks good