Medios-Macina/docs/PR_PLUGIN_AUTHORING.md

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.