Use consistent terminology to refer to a section in CAR file

The term "Section"[ is used](https://ipld.io/specs/transport/car/carv1/#format-description) to describe the repeated `` format that collectively make up the Data portion of a CAR file.

Use terminology consistent with docs in code to avoid confusion.
Example inconsistent terminology, where the term "block" is used to refer to a "Section":
https://github.com/ipld/go-car/blob/c4b9dc88fc62d07a453efc8ae76a341084096ae7/v2/blockstore/readonly.go#L184-L187