[Docs] Move API docs to the front page

This moves the links to the C++ and Python API docs up to the main page.
As of now the links are still broken [1], but at least this will prevent
the additional frustration of searching for the links only to find out
they're broken.

[1] http://lists.llvm.org/pipermail/lldb-dev/2019-April/014992.html

llvm-svn: 358928