fix dpl-docs

- use a stable release of dmd to build dpl-docs
create output dir if it doesn't yet exist
pass -version=CoreDdoc and filter out core.internal
upgrade dpl-docs dependencies
also add code.dlang.org link to new library docs
Merge pull request #706 from MartinNowak/dubLinkDPL

also add code.dlang.org link to new library docs
Merge pull request #705 from MartinNowak/fixDPLDocs

fix dpl-docs