Fix Sphinx warnings (broken line blocks, short title underline, html_static_path)

See the commit messages for details.  With this series, the only
outstanding Sphinx warnings are:

  /.../docs/index.rst:4: WARNING: nonlocal image URI found: https://api.travis-ci.org/wcdolphin/flask-cors.png?branch=master

and nine other nonlocal image warnings.