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A similar change to Encode and Decode function names came in
go-ipld-prime v0.9.0, so this keeps this library in sync.

Improved package docs a bit to talk about the new go-ipld-prime
LinkSystem, which is a bit clearer to refer to than before.

Target-of-opportunity fix some typos in docs.

The module was defined as github.com/ipld/go-codec-dagpb/dagpb, but the
repository is at github.com/ipld/go-codec-dagpb. This means that neither
can be resolved as a valid Go package import path; the former results in
a GitHub 404, and the latter results in a valid GitHub repository which
holds a different module.

Rod confirmed that the /dagpb suffix wasn't intentional, so just get rid
of it.

While at it, mod tidy.

Fixes: https://github.com/ipld/go-codec-dagpb/pull/3