Update docs in Academy to drop Alpine examples and replace with Wolfi

[erikaheidi]

Some of the examples reference Alpine. We need to update them to use Wolfi.

Drop references to alpine-based images since they are all wolfi based now, and try to clarify the ecosystem so users understand better what is wolfi and what are chainguard images.

[ltagliaferri]

Enforce docs can also include Chainguard Images instead of Alpine, such as Enforce Signing cc @SharpRake