Remove nested site generation in favor of just creating Scaladoc/Sxr docs.

* Do not delete versioned docs from sbt/website project
* Just sync api/sxr directories
* Remove all build-related information for old sphinx docs.

Replaces #1385
Remove old website in favor of new sbt/website project.
Merge pull request #1389 from sbt/wip/only-push-scaladoc

Remove nested site generation in favor of just creating Scaladoc/Sxr doc...
Fixes NullPointerException during update. Fixes #1484
Merged 0.13.5 to remove documents
Fix broken cross versioning semantics using +
Fix nightly push to go to snapshot repository.
Merge pull request #1499 from sbt/wip/fix-nightly-push

Fix nightly push to go to snapshot repository.
Merge pull request #1490 from sbt/wip/1484

Fixes NullPointerException during update. Fixes #1484
Merge pull request #1497 from sbt/wip/0.13.5-merge

0.13.5 merge
Merge pull request #1498 from sbt/wip/fix-broken-cross

Fix broken cross versioning semantics using +
JCenter and BintrayMavenRepository resolvers added