[dub-reference/subpackages] Basic content

[WebFreak001]

• example how to define and use
• mention how it differs from configurations inside a root package (individually usable, others can only use the subpackage and their dependencies in isolation)
• explain how Have_x_y version naming works here
• tell not to put examples inside subpackages, because they pollute the registry
• mention current registry problem that sub-packages are not easily discoverable, but may be fixed in the future by showing them in searches separately as well