Release: introduce magefile

[vankichi]

This PR has some new attempts.

• Introduce magefile
  • Due to the complex Makefile, I have migrated file I/O operation to ./magefile/magefile.go and refactored ./Makefile
  • we can update content files by make all or make TARGET_VER={X.Y.Z} all
• Add description.json for auto-update content files
  • description.json includes weight and description for each document file, which shows content information on the website.
• Add a new GitHub workflow, which runs by repository_dispatch and updates content files automatically.
  • Until now, we have needed to manually update when the document file or Vald version is updated, wasting the maintainers' time. So, I have created a new workflow that updates the content file triggered by the repository_dispatch from the vdaas/vald repository.

[ykadowak]

Will you change the logic of the megefile.go not to use unnecessary regex?

Other part looks good.

[vankichi]

@ykadowak
I have updated it. Could you please review it?

[hlts2]

LGTM!!