Add project example for subpackages

[vabenil]

The documentation for dub subpackages as the rest of the dub docs are lacking in examples.
Adding a repository with simple dub examples would massively help beginners getting into D and thereafter dub.

To be more specific my reason for opening this issue is because of the lack of mention of the file structure of a subpackage. In the dub subpackage documentation in this example:

Does not include that the source directory for component1 or component2 should be ./component/source/component/ whereas a beginner might assume that the correct path should ./component/source/.

Had there been an example dub project where I could have seen the correct file structure I would not have wasted hours with this issue.

[WebFreak001]

for current new docs deployment see https://deploy-preview-54--dub-registry.netlify.app/