Use consistent terminology to refer to a section in CAR file

[masih]

The term "Section" is used to describe the repeated <varint><cid><block> 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