docs: clarify metric input requirements

[GioiaZheng]

Why

RAGChecker users need a clear way to tell which input fields are required for each metric. The tutorial explains the metric groups, but it does not map metrics to their intermediate checks or clarify which metrics can be used without reference answers.

Refs #29.

What changed

• Add a metric input requirements table to the English tutorial.
• Map each metric to the intermediate claim-checking result it depends on.
• Clarify that faithfulness is reference-free in its computation, while the other listed metrics rely on gt_answer directly or indirectly.
• Note how to interpret missing fields versus intentionally empty intermediate results.

Validation

• Reviewed the table against METRIC_REQUIREMENTS and the metric computation paths.
• Ran git diff --check.
• Checked added lines for unintended attribution/tooling text.

Risk / follow-up

Low risk; documentation-only change. If this structure looks useful, the same summary could be mirrored in the README later.