docs.source.conf: Add avocado top dir to sys.path

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado: Add API documentation

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #21 from avocado-framework/api-docs

Api docs