Clarify that marking certain member variables const can hinder performance

In the 03-Style document, I read the following sentence:

> If the member variable is not expected to change after the initialization, then mark it const.

However, we know that if, for example, we mark a container as `const`, we prevent it from being moved, possibly hindering performance. Marking fields as `const` is useful to enforce invariants; however, it "interferes" with performance. If both const correctness and performance are important, one could leave the field non-const, make it private and define a field getter.

首先定位 03-Style 文件，並檢視其中引用的關於將未變更的成員變數標記為 const 的指引。釐清文件所述的 const 正確性與移動容器之間的取捨，包括 issue 中描述的 private 欄位 getter 選項，然後驗證轉譯後的文件。

由索引模型根據 Issue 內容生成。