Medios-Macina/docs/PR_PROVIDER_AUTHORING.md

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.