Vet and update the go/build documentation.

[vaikas]

There's been some changes (to my untrained eye at least) for the go/build.

https://github.com/chainguard-dev/melange/blob/main/docs/PIPELINES-GO.md

This tracks making sure we vet the existing documentation and updates with any new features.

For example ${{targets.contextdir}} and previously necessary mv that's no longer needed.