All-new improved, better readable, maintainable DUB documentation

[WebFreak001]

bump (for email readers, URL of this thread: 
https://forum.dlang.org/post/ojoiwbcftqsxbsvivghz@forum.dlang.org)

- added some new pages with new content
- github issues still contain information for contributors that 
want to help improving the documentation
- on https://docs.webfreak.org it now shows links to the issues 
with content hints and suggestions on each page, so that you can 
easily find pages to work on