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...
Merged 0.13.5 to remove documents
Merge pull request #1497 from sbt/wip/0.13.5-merge

0.13.5 merge