Move Harbor-CLI out of sandbox

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 https://github.com/goharbor/website/pull/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.