docs: Show TOC for GettingStarted.rst.

This is a pretty lengthy document, so put the table of contents in your
face so that it's easier to scope out the content.

This document is a mess currently and needs to be
refactored/revised/split-up.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@170646 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed