Generate an API reference with mkdocstrings #137

We have fourteen hand-written guides and no generated API reference. Every comparable project has one (llm-guard, guardrails, NeMo-Guardrails), and it is the page people search for when they want to know what a method actually takes.

The good news is the groundwork is already done. 39 of the 40 names in unplug.__all__ have docstrings. They are one-line summaries rather than full Args/Returns blocks, but combined with the type annotations that is enough to render a genuinely useful reference. No docstring rewrite is needed first.

get_correlation_id is the one name with no docstring.

What to do:

1. Add mkdocstrings[python] to docs-requirements.txt and the plugin to mkdocs.yml.
2. Add a docs/reference/ section with a page per major type: Guard, ScanResult, Finding, GuardConfig, and the enums.
3. Add a Reference nav section.

Do not use unplug.core.* paths in the reference. docs/PUBLIC_API.md defines what is public, and the reference should cover that surface and nothing below it.

Acceptance:

• Every name in unplug.__all__ is reachable from the reference nav.
• mkdocs build --strict passes.
• Signatures render with types, and the summary line appears under each.

Not a first issue.