MANIFEST.in: add entire docs folder (not just rst)

When a release gets created the docs folder is missing the sphinx Makefile, config, static, and template folders which means it's not possible to build the docs with sphinx. This patch adds the missing pieces.
MANIFEST.in: exclude docs/_build from packaging
Merge pull request #10 from jtriley/patch-1

MANIFEST.in: add entire docs folder (not just rst)