Language specification sources moved to dmd repository

[Dennis]

The Ddoc sources for the language specification have been moved 
from https://github.com/dlang/dlang.org to 
https://github.com/dlang/dmd

It's common for changes in DMD to also require a change in the 
documentation, but this is often neglected because it's annoying 
to create 2 separate Pull Requests and keep them in sync.

Now it's a lot easier to create a complete language change in 1 
PR, updating not only the implementation, test suite, and 
changelog like before, but also the documentation.

For readers of the specification, nothing changed. It's still at: 
https://dlang.org/spec/spec.html

The only difference in the html content is that the "Report a 
bug" and "Improve this page" texts on the top right will send you 
to the new location. (If you're seeing an old cached copy of the 
website still linking to github/dlang.org press Ctrl+F5 for a 
hard refresh)

When I brought this idea up in the DLF monthly meeting last 
Friday, I got thumbs up and Atila even expressed desire to also 
move Phobos into the dmd repository, but there's no plan for that 
yet.