Initial docs directory

Adding a new docs directory, with files auto-generated by sphinx-quickstart,
for user documentation for the various tools in the clang-tools-extra
repository.

Author: Jack Yang <jack.yang@intel.com>
Reviewers: gribozavr, silvas


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@175118 91177308-0d34-0410-b5e6-96231b3b80d8