Add feature importance report for crop recommendation model

[sardorcodev]

Add a feature importance report for the crop recommendation model.

Scope:

• Generate feature importance from the candidate training workflow.
• Save report under docs/ml/metrics/.
• Document limitations of feature importance for MVP model interpretation.
• Do not claim agronomic reliability from feature importance alone.

Acceptance criteria:

• Feature importance report exists.
• MODEL_CARD.md links to the report.
• Training workflow can regenerate the report.