Less broadcast for numpy exercise

Currently the indexing in the numpy example and exercise revolve a lot around broadcasting.

This is not necessary very useful. Other kind of indexing, like selection with boolean mask or range selection may be more interesting.

找出 issue 中提到的 numpy 範例和練習，並比較它們在多大程度上依賴 broadcasting。修改教材，改為強調布林遮罩選取或範圍選取，然後確認範例和練習對 workshop 來說仍然一致。

由索引模型根據 Issue 內容生成。