it is time-consuming to rebuild the python bindings every time we
rebuild the document, it'd be ideal if we could just build document.
in this change, in addition to "html" and "doc", "livehtml" argument is
now supported by build-doc script, so one can just use
./build-doc livehtml
to build and start a web server. whenever a change in doc/ is detected,
the document is rebuilt. for more details, see
https://pypi.org/project/sphinx-autobuild/