The dub documentation is now on dub.pm

[Seb]

tl;dr: The dub documentation got split from the dub-registry 
repository and now lives on https://dub.pm 
(https://github.com/dlang/dub-docs).

Motivation:
-----------

- makes building the dub-registry faster
- people hosting a mirror or a private instance don't need to 
host the documentation too
- allows us to use a separate, more modern design for the dub 
docs (proposals are welcome!)
- makes it easier for people contributing the docs (less code and 
dependencies to build)
- allows us to remove the menu header at code.dlang.org and 
replace it with a nice, modern searchbar
- makes the deployment for the documentation easier (they are 
static
HTML after all) and we have already setup continuous deployment 
for the documentation (via Netlify)

As always, ideas and PRs to make DUB's documentation better are 
welcome.

PS: Of course all old documentation links will redirect to dub.pm