Revisit `file.PackFiles()`

[Wwwsylvia]

file.PackFiles() allows users to add files and pack a manifest quickly. But the current design might not be perfect. We should revisit it and think about if we should keep/refactor/deprecate it in oras-go v2.0.0.

https://github.com/oras-project/oras-go/blob/1969551cc3c77b46f09090f00360e8465871fe8f/content/file/file.go#L429-L443

[Wwwsylvia]

One use case of this function can be found here.

[shizhMSFT]

@Wwwsylvia I suggest removing this function, and adding an example instead.

[sajayantony]

+1 to an example with just one file add to show how someone can generate a manifest and to keep the lines of code less.