Un-revert "[helloworld] Modernised project structure"

This reverts commit 64b3c071c4255eb4207d7f6ce76b4472c4360375.
Un-revert "[helloworld] Added simple activator-based example"

This reverts commit 29935c3d7bb81e9b80ef797a967cfde7705792a0.
Un-revert "[helloworld] Converted to file: URI repopaths"

This reverts commit d1e6f8aadb07e21563e89037df2d9f4d86ab6d5b.
Un-revert "[helloworld] Updated for new-style repopaths"

This reverts commit b400a95554cf4eafed0e909c92ec347de68e7777.
[helloworld] Updated system doc for DS