docs: Fix long standing linking antipattern.

Before we learned about :doc:, we used :ref: and put a dummy link at the
top of each page. Don't do that anymore.

This fixes PR14891 as a special case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172162 91177308-0d34-0410-b5e6-96231b3b80d8