PR Title: docs: Add plugin authoring doc, examples, and tests

Summary:
- Add `docs/plugin_authoring.md` describing the strict `ResultModel`-based plugin adapter pattern, `ColumnSpec` usage, `selection_fn`, and `TablePluginMixin` for HTML table scraping.
- Link the new doc from `docs/result_table.md`.
- Add plugin-author example coverage to validate the example plugin integration with the registry.

Why:
- Provide a short, focused Quick Start to help contributors author plugins that integrate with the strict ResultTable API.

Testing:
- New tests pass locally (plugin-related subset).

Notes:
- The change is documentation-first and non-functional, with tests ensuring examples remain valid.
