Add configurable Azure Document Intelligence analysis features via CLI and kwargs

[kei-yamazaki]

Summary

This PR makes Azure Document Intelligence analysis features configurable while preserving the existing default behavior.

What changed

• Added CLI support for custom DI analysis features:
  • --docintel-feature (repeatable and comma-separated)
• Added Python kwargs support:
  • MarkItDown(..., docintel_features=[...])
  • per-call override via convert(..., docintel_features=[...])
• Switched feature handling to SDK constants (DocumentAnalysisFeature) for type safety
• Added normalization for common feature input forms (e.g. FORMULAS, ocr_high_resolution, DocumentAnalysisFeature.STYLE_FONT)
• Added validation for invalid feature names (raises ValueError)
• Kept default behavior when features are not specified:
  • OCR-capable formats: FORMULAS, OCR_HIGH_RESOLUTION, STYLE_FONT
  • .docx, .pptx, .xlsx, .html: no analysis features
• Updated README:
  • documented CLI feature option
  • documented default feature behavior when not specified
  • removed Python feature-customization sample section per latest doc direction

Why

• Users need to control DI analysis features depending on accuracy/cost/performance requirements.
• The previous implementation had fixed features and limited flexibility.
• Using SDK constants improves safety and reduces string-based errors.