[feature] Document that signed provenance replaces signing artifacts

[gabibguti]

Is your feature request related to a problem? Please describe.
It is not clear through the SLSA 3 Go Builder documentation if it signs both the provenance and the artifacts. As I understand, SLSA 3 Go Builder signs the provenance and does not sign the artifacts. That's because a signed provenance replaces the need of signed artifacts since the hash of the artifacts is included in the provenance subjects. There may be other reasons to sign the artifacts but not to "prove" the integrity of the artifacts as it's already done by the signed provenance.

Describe the solution you'd like
Document that SLSA 3 Go Builder signs the provenance and does not sign the artifacts, and that there's no need to sign the artifacts since the hash of the artifacts is included in the provenance subjects.

Describe alternatives you've considered
None.

Additional context
None.