refactor: clean up deprecated code and update plugin docs

- Remove outdated Phase comments from _options.py and cli.py
- Remove unused methods from PluginOptionRegistry:
  - get_extended_options_model() - replaced by __getattr__ in OCROptions
  - map_legacy_options() - unused
  - validate_plugin_options() - unused
- Update plugin documentation to document register_options hook
- Add documentation for nested plugin option access pattern

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/ocrmypdf/cli.py

@@ -462,10 +462,6 @@ def namespace_to_options(ns) -> OCROptions:
     if 'work_folder' in extra_attrs and 'input_file' not in known_fields:
         known_fields['input_file'] = '/dev/null'  # Placeholder
 
-    # Handle backward compatibility for plugin options
-    # Map CLI arguments to the appropriate fields for now
-    # In Phase 2, this will be handled by plugin option models
-
     instance = OCROptions(**known_fields)
     instance.extra_attrs = extra_attrs
     return instance