Implement sticky sidebar support.

It would be nice to have a sticky sidebar when displaying some long text.

This implementation is derived from sphinx's [default theme](https://bitbucket.org/birkenfeld/sphinx/src/b6065ae5d7c055e07686c74e789a00589d20a320/sphinx/themes/default/static/default.css_t?at=default#cl-74), it just works.