chore: separate docs:generate

[knqyf263]

Description

The mage command is quite slow now since it imports github.com/aquasecurity/trivy to generate docs. Mage tries to compile magefiles/magefile.go, then also compiles the trivy packages. All other targets, such as mage fmt and mage lint, don't need the trivy packages, and they should be fast.

This PR separates mage docs:generate into a different file with a custom tag so that other targets will not import Trivy.

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).