[dub-guide/registry] Basic content

[WebFreak001]

• [ ] What the registry is
• [ ] Where it is hosted / who owns it
• [ ] Source code to registry: https://github.com/dlang/dub-registry
• [ ] Tell how the registry is not the CLI and how it's a kind-of-optional component
• [ ] Show how to use the basic web features, link to publishing packages
• [ ] Link to https://docs.webfreak.org/dub-reference/registries/ for build/development instructions