Document how to indent YAML block sequences

[user01010111]

Addresses #115 with a tested recipe using the existing Dumper keyword. It explains why indent=4 alone leaves sequences indentless and why the override must subclass Python's yaml.SafeDumper, including when LibYAML is installed.

The example checks exact indentation and round-trips nested metadata and content. It retains safe serialisation and explains that source formatting is not preserved. No dependencies, formatting defaults or handler APIs change.

Validation: 68 tests passed on Python 3.13 with PyYAML 6.0.3 and LibYAML; mypy and package build passed. The HTML docs build succeeds with the same 12 warnings as the base revision. All eight recipe doctests pass; the full Sphinx doctest run still has the existing API setup failure caused by >>> import frontmatter inside a testsetup block.