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.

Start by locating the 03-Style document and reviewing the quoted guidance about marking unchanged member variables const. Clarify the documented trade-off between const correctness and moving containers, including the private-field getter option described in the issue, then verify the rendered document.

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