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

Summary:
- Add `docs/provider_authoring.md` describing the strict `ResultModel`-based provider adapter pattern, `ColumnSpec` usage, `selection_fn`, and `TableProviderMixin` for HTML table scraping.
- Link new doc from `docs/result_table.md`.
- Add `tests/test_provider_author_examples.py` to validate `Provider/example_provider.py` and `Provider/vimm.py` integration with the registry.

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

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

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