Move Harbor-CLI out of sandbox

[bupd]

harbor-cli should be made more visible as an official sub-project.

Why:

• harbor-cli is already used widely
• it has passed 130k+ downloads
• maintainership should be explicit and public

Proposal:

• merge Add documentation for Harbor CLI configuration and encryption management website#645 to make harbor-cli more visible on the website
• define a MAINTAINERS.md in harbor-cli
• follow the CNCF-style maintainer pattern so sub-project maintainers are clearly listed
• use this as the step to move harbor-cli out of the Harbor sandbox

This would better reflect current adoption and project maturity.

[bupd]

Draft PR opened for the maintainer file:

• docs: add MAINTAINERS.md #741

This adds MAINTAINERS.md with the current sub-project maintainers.