.gitignore: ignore docs/_build

That is where the documentation gets built to.

I'm pretty much clueless about SVN; I would appreciate if someone more
knowledgeable about SVN made the equivalent change.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@176417 91177308-0d34-0410-b5e6-96231b3b80d8