[dub-reference/output_target] Basic content

[WebFreak001]

• describe how the binary output path is generated by default
• link to build settings affecting the output
• describe what unittest does by default, how it behaves when overriden and if it's possible to somehow restore that behavior
• tell about how dub describe can be used to get the values programmatically as well (but no way to directly get the binary output yet)
• describe that there may be multiple outputs depending on the target
  • give samples (on windows PDB and DLL related files are emitted for example, check DUB source - my object file output PR might touch this as well)
• List default file extensions per platform
• Give easy copy-pasteable snippet for bin target output, like commonly used in some packages (targetPath "bin")