Preparation for v0.2.0 release

[stefanprodan]

As discussed in #367 we've decided to rename:

• the toolkit repository to flux2
• the gotk CLI binary to flux
• the gotk-system namespace to flux-system

Tasks:

• make v0.2.0 release of the toolkit APIs and controllers
• rewrite the readme and docs to explain Flux v2
• rename the repository
• rename the CLI pkg, system namespace in code, goreleaser, brew formula
• rename the CLI and system namespace in docs
• rewrite references to "GitOps Toolkit"
• make v0.2.0 release of the flux CLI

[stefanprodan]

🎉 https://github.com/fluxcd/flux2/releases/tag/v0.2.0