[clang-doc] Add stylesheet to generated html docs

A default css stylesheet is included for docs generated in html format.

Differential Revision: https://reviews.llvm.org/D64539

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@367056 91177308-0d34-0410-b5e6-96231b3b80d8
10 files changed