A few minor updates based on feedback from Justin and a few things I thought were...

A few minor updates based on feedback from Justin and a few things I thought were missing.

* Moved autotools configure & build example out of "Getting Started Quickly (A Summary)" and into BuildingLLVMWithAutoTools.
* Removed the annotations that CMake is the recommended process and Autotools is alternate.
* Added brief documentation about build targets under "Getting Started Quickly..."
* Added Overview text to BuildingLLVMWithAutotools
* Fixed up a broken link.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232278 91177308-0d34-0410-b5e6-96231b3b80d8