decree-docs: documentation, example outputs, freshness CI

[zeevdr]

Description

User-facing documentation for the tool: a README in contrib/decree-docs, a page in the MkDocs site, and committed example outputs (md, mdx, html, json) generated from a fixture schema that covers every field type and option. A CI job regenerates the examples and fails on diff, following the generated-docs freshness pattern established in #880.

Acceptance criteria

• README documents all formats, flags, config keys, and theming options.
• MkDocs site page added and listed in the mkdocs.yml nav.
• Committed example outputs for every format, generated from a full-coverage fixture schema.
• Test coverage: CI regen-diff job fails when committed examples are stale.