docs: build independent document website

[imeoer]

• Website: https://nydus.dev/docs/
• Reference style: https://d7y.io/docs/
• Goal: Reorganize existing documentation https://github.com/dragonflyoss/image-service/tree/master/docs to https://github.com/dragonflyoss/image-service-website/tree/main/docs
• Structure: As shown below